Fixups to "Round-robin repository storage"

## What does this MR do?

* Simplifies a method in application_settings.rb
* Correctly marks a migration as needing downtime
* Documents the requirement for renamed columns to be 

## Are there points in the code the reviewer needs to double check?

Should any of these changes be split out? Ideally we'd get this into the same point release as !7273

## Why was this MR needed?

Post-facto review of !7273

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [X] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [X] API support added
- Tests
  - [X] Added for this feature/bug
  - [x] All builds are passing
- [X] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [X] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [X] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [X] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Related to #24059

/cc @yorickpeterse @rspeicher

See merge request !7287