Fix `datetime_with_timezone` multiple problems

1. The first timestamp column on each table in MySQL is NOT NULL and defaults to CURRENT_TIMESTAMP on update

This is a MySQL "feature": https://bugs.mysql.com/bug.php?id=75098

This can cause unexpected master failures since we don't test MySQL on every branch, but we do on master, like in https://gitlab.com/gitlab-org/gitlab-ee/issues/3150#note_37391138.

Edit: This was worked around in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13560, but it is not working for @vsizov in the Rails 5 upgrade.

Update: GitLab no longer supports MySQL.

2. Current ActiveRecord monkeypatch doesn't always generate schema with datetime_with_timezone

See @yorickpeterse's example in this comment

I have a migration that changes the type of two columns from datetime to datetime_with_timezone but they still get dumped as t.datetime in db/schema.rb

Update: GitLab only supports Postgres and we use db/structure.sql, so we don't need to generate db/schema.rb with datetime_with_timezone. (We do have a ee/db/geo/schema.rb so perhaps it's possible there, but we haven't needed to do this kind of migration there. Also, we plan to migrate to structure.sql as well #323806)

3. Current ActiveRecord monkeypatch doesn't work in Rails 5

After Rails 5 upgrade, the schema will no longer be generated with datetime_with_timezone columns, which will probably cause some confusion, and eventually inconsistent development/test DB setups.

This was due to the following:

Comment https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12841#note_35154488

FYI In rails 5, they significantly rewrote DB adapters so this hack https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config/initializers/active_record_data_types.rb#L64 does not work anymore...

Comment https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12841#note_35174984

I reverted the origin version of the config/initializers/active_record_data_types.rb monkey patch. It should unblock this MR but re-introduce schema dump problems, see the git log https://gitlab.com/gitlab-org/gitlab-ce/commits/master/config/initializers/active_record_data_types.rb

Update: GitLab only supports Postgres and we use db/structure.sql, so we don't need to generate db/schema.rb with datetime_with_timezone. (We do have a ee/db/geo/schema.rb but I am unaware of this problem there. Also, we plan to migrate to structure.sql as well #323806)