Added helper methods for database migrations
These helpers can ultimately be used to write migrations that don't require downtime. Right now this is still a WIP/giant hack, I'll update this MR once it's nearing completion.
See #15464 (closed) for more information.
Obviously test things
- Find a way to update Rails migration generators so migrations automatically include this module
- Automatically insert a comment in generated migrations to remind developers to actually use these methods
- Test these methods on a table with plenty of data (e.g. something the size of GitLab.com's projects table)