Add dedicated group relation to user & group classes

What does this MR do?

This MR creates a database structure for storing account information for provisioned dedicated group accounts. See more in https://gitlab.com/gitlab-org/gitlab-ee/issues/6260

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Database checklist

• Conforms to the database guides

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
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides

Migration outputs:

== 20190222105948 AddUserManagingGroupRelation: migrating =====================
-- add_column(:users, :managing_group_id, :integer)
   -> 0.0030s
== 20190222105948 AddUserManagingGroupRelation: migrated (0.0031s) ============

== 20190222110418 AddUserManagingGroupRelationFk: migrating ===================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:users, :managing_group_id, {:algorithm=>:concurrently})
   -> 0.0096s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- add_index(:users, :managing_group_id, {:algorithm=>:concurrently})
   -> 0.0082s
-- execute("RESET ALL")
   -> 0.0004s
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:users)
   -> 0.0032s
-- execute("ALTER TABLE users\nADD CONSTRAINT fk_a4b8fefe3e\nFOREIGN KEY (managing_group_id)\nREFERENCES namespaces (id)\nON DELETE SET NULL\nNOT VALID;\n")
   -> 0.0100s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- execute("ALTER TABLE users VALIDATE CONSTRAINT fk_a4b8fefe3e;")
   -> 0.0033s
-- execute("RESET ALL")
   -> 0.0005s
== 20190222110418 AddUserManagingGroupRelationFk: migrated (0.0370s) ==========