Starting with GitLab 9.1.0 we will no longer allow downtime migrations
unless absolutely necessary. This commit updates the various developer
guides and adds code that is necessary to make zero downtime migrations
less painful.
These new checks can be used to check if migrations require downtime or
not (as tagged by their authors). In CI this compares the current branch
with master so migrations added by merge requests are automatically
verified.
To check the migrations added since a Git reference simply run:
bundle exec rake gitlab:db:downtime_check[GIT_REF]
