Capture migration output as an artifact

We currently require the author of an MR to add migration output to the MR description.

Add the output of both migrating and rolling back for all migrations into the MR description.

In an MR, this looks like:

We should capture this information when we run migrations and link to these artifacts in the comment. Besides being automated, it will also be nice to have this information in the same place as the other info we collect.

Steps

• Save up migration output to a file
• Prep the gitlab project for down migrations (gitlab-org/gitlab!71111 (merged))
• Start using the new :up and :down rake tasks in the notifier
• Remove the old gitlab:db:migration_testing rake task
• Save down migration output to a file