Update PagesDomains data model for serverless domains

What does this MR do?

Update data model of PagesDomains in response to discussion in !21222 (merged).

• Add usage enum to distinguish between serverless and pages domains
• Replace unique index on domain with unique index on [domain, wildcard] so that the wildcard and non-wildcard variants of a domain can coexist
• Rename domain_type to scope to be more semantic

/cc @ayufan @krasio @grzesiek @mattkasa

Output of migrations

== 20200128184209 AddUsageToPagesDomains: migrating ===========================
-- transaction_open?()
   -> 0.0000s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- transaction()
-- add_column(:pages_domains, :usage, :integer, {:default=>nil, :limit=>2})
   -> 0.0013s
-- change_column_default(:pages_domains, :usage, 0)
   -> 0.0039s
   -> 0.0083s
-- columns(:pages_domains)
   -> 0.0020s
-- transaction_open?()
   -> 0.0000s
-- exec_query("SELECT COUNT(*) AS count FROM \"pages_domains\"")
   -> 0.0018s
-- change_column_null(:pages_domains, :usage, false)
   -> 0.0006s
-- execute("RESET ALL")
   -> 0.0003s
== 20200128184209 AddUsageToPagesDomains: migrated (0.0136s) ==================

== 20200129034515 UpdateIndexesOfPagesDomainsAddUsageDomainWildcardRemoveDomain: migrating 
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:pages_domains, :usage, {:algorithm=>:concurrently})
   -> 0.0051s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- add_index(:pages_domains, :usage, {:algorithm=>:concurrently})
   -> 0.0055s
-- execute("RESET ALL")
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:pages_domains, [:domain, :wildcard], {:unique=>true, :algorithm=>:concurrently})
   -> 0.0032s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- add_index(:pages_domains, [:domain, :wildcard], {:unique=>true, :algorithm=>:concurrently})
   -> 0.0023s
-- execute("RESET ALL")
   -> 0.0002s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:pages_domains, :domain, {:algorithm=>:concurrently})
   -> 0.0037s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- remove_index(:pages_domains, {:algorithm=>:concurrently, :column=>:domain})
   -> 0.0072s
-- execute("RESET ALL")
   -> 0.0002s
== 20200129034515 UpdateIndexesOfPagesDomainsAddUsageDomainWildcardRemoveDomain: migrated (0.0289s) 

== 20200129035446 RenamePagesDomainsDomainTypeToScope: migrating ==============
-- transaction_open?()
   -> 0.0000s
-- columns(:pages_domains)
   -> 0.0024s
-- add_column(:pages_domains, :scope, :integer, {:limit=>2, :precision=>nil, :scale=>nil})
   -> 0.0009s
-- change_column_default(:pages_domains, :scope, "2")
   -> 0.0027s
-- transaction_open?()
   -> 0.0000s
-- exec_query("SELECT COUNT(*) AS count FROM \"pages_domains\"")
   -> 0.0006s
-- change_column_null(:pages_domains, :scope, false)
   -> 0.0007s
-- indexes(:pages_domains)
   -> 0.0041s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:pages_domains, ["scope"], {:unique=>false, :name=>"index_pages_domains_on_scope", :length=>{}, :order=>{}, :using=>:btree, :algorithm=>:concurrently})
   -> 0.0042s
-- execute("SET statement_timeout TO 0")
   -> 0.0003s
-- add_index(:pages_domains, ["scope"], {:unique=>false, :name=>"index_pages_domains_on_scope", :length=>{}, :order=>{}, :using=>:btree, :algorithm=>:concurrently})
   -> 0.0028s
-- execute("RESET ALL")
   -> 0.0003s
-- foreign_keys(:pages_domains)
   -> 0.0024s
-- quote_table_name(:pages_domains)
   -> 0.0000s
-- quote_column_name(:domain_type)
   -> 0.0000s
-- quote_column_name(:scope)
   -> 0.0000s
-- execute("CREATE OR REPLACE FUNCTION trigger_5e971e2e2c26()\nRETURNS trigger AS\n$BODY$\nBEGIN\n  NEW.\"scope\" := NEW.\"domain_type\";\n  RETURN NEW;\nEND;\n$BODY$\nLANGUAGE 'plpgsql'\nVOLATILE\n")
   -> 0.0055s
-- execute("DROP TRIGGER IF EXISTS trigger_5e971e2e2c26\nON \"pages_domains\"\n")
NOTICE:  trigger "trigger_5e971e2e2c26" for relation "pages_domains" does not exist, skipping
   -> 0.0004s
-- execute("CREATE TRIGGER trigger_5e971e2e2c26\nBEFORE INSERT OR UPDATE\nON \"pages_domains\"\nFOR EACH ROW\nEXECUTE PROCEDURE trigger_5e971e2e2c26()\n")
   -> 0.0008s
== 20200129035446 RenamePagesDomainsDomainTypeToScope: migrated (0.0547s) =====

== 20200129035708 CleanupRenamePagesDomainsDomainTypeToScope: migrating =======
-- execute("DROP TRIGGER IF EXISTS trigger_5e971e2e2c26 ON pages_domains")
   -> 0.0007s
-- execute("DROP FUNCTION IF EXISTS trigger_5e971e2e2c26()")
   -> 0.0005s
-- remove_column(:pages_domains, :domain_type)
   -> 0.0017s
== 20200129035708 CleanupRenamePagesDomainsDomainTypeToScope: migrated (0.0221s)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

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• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

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• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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