Feature to create directly addressed Todos when mentioned in beginning

Closes #24976

See merge request !7926

Add member: Always return 409 when a member exists

Closes #20732

See merge request !9093

Update Rubocop to Ruby 2.3

See merge request !8994

Add method for creating foreign keys concurrently

See merge request !9069

Rename Files::DeleteService to Files::DestroyService

See merge request !9110

This method allows one to create foreign keys without blocking access to
the source table, but only on PostgreSQL.

When creating a regular foreign key the "ALTER TABLE" statement used for
this won't return until all data has been validated. This statement in
turn will acquire a lock on the source table. As a result this lock can
be held for quite a long amount of time, depending on the number of rows
and system load.

By breaking up the foreign key creation process in two steps (creation,
and validation) we can reduce the amount of locking to a minimum.
Locking is still necessary for the "ALTER TABLE" statement that adds the
constraint, but this is a fast process and so will only block access for
a few milliseconds.

This reverts commit b7c5ca499d9c26494736d92505116bbb294c63d6, reversing
changes made to 9745c98bb77a1a1ddd964f58cc4d058a665eb9ee.

Small improvements for Cycle Analytics

See merge request !9126

Optionally make users created via the API set their password

Closes #1051

See merge request !8957

Reason for renaming is to comply with naming convention of services in
codebase.