Add indexes to all geo event foreign keys

Merged Toon Claes requested to merge tc-add-missing-geo-event-indexes into master

What does this MR do?

Because the Geo Event tables do a on_delete: :cascade, the foreign keys in geo_event_log need to have an index. Otherwise the deletion would be very slow.

See https://docs.gitlab.com/ee/development/migration_style_guide.html#adding-foreign-key-constraints

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/7955

Does this MR meet the acceptance criteria?

Edited by Toon Claes