Add index on namespaces lower(name) for UsersController#exists

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Fixes #41340 (closed)

Database Checklist

When adding migrations:

• Updated db/schema.rb
• Added a down method so the migration can be reverted
• Added the output of the migration(s) to the MR body
• Added tests for the migration in spec/migrations if necessary (e.g. when migrating data)

When adding or modifying queries to improve performance:

• Included data that shows the performance improvement, preferably in the form of a benchmark
• Included the output of EXPLAIN (ANALYZE, BUFFERS) of the relevant queries

When adding foreign keys to existing tables:

• Included a migration to remove orphaned rows in the source table before adding the foreign key
• Removed any instances of dependent: ... that may no longer be necessary

When adding tables:

• Ordered columns based on the Ordering Table Columns guidelines
• Added foreign keys to any columns pointing to data in other tables
• Added indexes for fields that are used in statements such as WHERE, ORDER BY, GROUP BY, and JOINs

When removing columns, tables, indexes or other structures:

• Removed these in a post-deployment migration
• Made sure the application no longer uses (or ignores) these structures

General Checklist

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

$ bundle exec rake db:migrate
== 20171220191323 AddIndexOnNamespacesLowerName: migrating ====================
-- index_exists?(:namespaces, {:name=>:index_on_namespaces_lower_name})
   -> 0.0095s
-- execute("CREATE INDEX CONCURRENTLY index_on_namespaces_lower_name ON namespaces (LOWER(name));")
   -> 0.0165s
== 20171220191323 AddIndexOnNamespacesLowerName: migrated (0.0262s) ===========

$ bundle exec rake db:rollback
== 20171220191323 AddIndexOnNamespacesLowerName: reverting ====================
-- remove_index(:namespaces, {:name=>:index_on_namespaces_lower_name})
   -> 0.0042s
== 20171220191323 AddIndexOnNamespacesLowerName: reverted (0.0044s) ===========

This should address this query:

gitlabhq_production=# explain SELECT  "namespaces".* FROM "namespaces" WHERE (lower(path) = 'username' OR lower(name) = 'username') LIMIT 1;
                                               QUERY PLAN                                               
--------------------------------------------------------------------------------------------------------
 Limit  (cost=0.00..12.36 rows=1 width=189)
   ->  Seq Scan on namespaces  (cost=0.00..129220.82 rows=10456 width=189)
         Filter: ((lower((path)::text) = 'username'::text) OR (lower((name)::text) = 'username'::text))