Populate user_highest_roles table

What does this MR do?

Part of #118592 (closed)

Depends on !27231 (merged) and !28087 (merged) in order to keep the table up-to-date.

Populate the user_highest_roles table with a background migration which schedules ~530 jobs with a 5 minutes gap to create an entry for each active User (~5.3 million).

This is a suggestion from this comment in order to resolve performance issues when querying too much data.

Adds an index to the users table:

Up migration:

== 20200317110602 AddMigratingUserHighestRolesTableIndexToUsers: migrating ====
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:users, :id, {:where=>"state = 'active' AND user_type IS NULL AND bot_type IS NULL AND ghost IS NOT TRUE", :name=>"index_for_migrating_user_highest_roles_table", :algorithm=>:concurrently})
   -> 0.0090s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- add_index(:users, :id, {:where=>"state = 'active' AND user_type IS NULL AND bot_type IS NULL AND ghost IS NOT TRUE", :name=>"index_for_migrating_user_highest_roles_table", :algorithm=>:concurrently})
   -> 1.1810s
-- execute("RESET ALL")
   -> 0.0003s
== 20200317110602 AddMigratingUserHighestRolesTableIndexToUsers: migrated (1.1906s) 

Down migration:

== 20200317110602 AddMigratingUserHighestRolesTableIndexToUsers: reverting ====
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:users, :id, {:name=>"index_for_migrating_user_highest_roles_table", :algorithm=>:concurrently})
   -> 0.0105s
-- execute("SET statement_timeout TO 0")
   -> 0.0002s
-- remove_index(:users, {:name=>"index_for_migrating_user_highest_roles_table", :algorithm=>:concurrently, :column=>:id})
   -> 0.0175s
-- execute("RESET ALL")
   -> 0.0002s
== 20200317110602 AddMigratingUserHighestRolesTableIndexToUsers: reverted (0.0287s) 

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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

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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• Label as security and @ mention @gitlab-com/gl-security/appsec
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