Review database indexes missing on GitLab.com

The following indexes are expected (per the codebase), but not present on GitLab.com:

> design_management_versions	public.index_design_management_versions_on_author_id	CREATE INDEX index_design_management_versions_on_author_id ON public.design_management_versions USING btree (author_id) WHERE (author_id IS NOT NULL)
> environments	public.index_environments_on_auto_stop_at	CREATE INDEX index_environments_on_auto_stop_at ON public.environments USING btree (auto_stop_at) WHERE (auto_stop_at IS NOT NULL)

This issue is about re-creating those indexes to align GitLab.com with expectations.