We need to make sure merge_requests.source_project_id allows NULL values
_before_ updating rows as otherwise this will lead to a NOT NULL
constraint failing.

This adds various foreign keys and indexes to the "merge_requests" table
as outlined in https://gitlab.com/gitlab-org/gitlab-ce/issues/31825.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/31825

Adds Rubocop rule for line break after guard clause

Closes #18040

See merge request gitlab-org/gitlab-ce!15188

Restrict update column in batches for large tables

See merge request gitlab-org/gitlab-ce!15458

add_column_with_default is implemented in terms of update_column_in_batches, but
update_column_in_batches can be used independently. Neither of these should be
used on the specified large tables, because they will cause issues on large
instances like GitLab.com.

This also ignores the cop for all existing migrations, renaming
AddColumnWithDefaultToLargeTable where appropriate.

Adds a rubocop rule (with autocorrect) to ensure line break after guard clauses.

This adds various foreign key constraints, indexes, missing NOT NULL
constraints, and changes some column types from timestamp to
timestamptz.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/31811