Resolve "Introduce round-robin project creation to spread load over multiple shards"
## What does this MR do?
Allow multiple shards to be enabled in the admin settings page, balancing project creation across all enabled shards.
## Are there points in the code the reviewer needs to double check?
* `f.select ..., multiple: true` isn't the most beautiful UI in the world, but switching to `collection_check_boxes` (or a facsimile thereof) isn't trivial
* Should `pick_repository_storage` be a method of `ApplicationSetting`, or `Project`? It's going to accrete logic over time so perhaps it should be its own class already?
* This is written to avoid the need for a database migration, so it is`serialize :repository_storage` without `, Array`. This is tested, but alternatives include:
* Add a database migration
* Write a custom Coder that will accept a String or Array in `load` and always `dump an Array.
## Why was this MR needed?
## Screenshots (if relevant)
## Does this MR meet the acceptance criteria?
- [X] [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
- [ ] 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?
Closes#24059
See merge request !7273
5ef2bd192a