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
Issues atom feed url reflect filters on dashboard
Makes the issue atom feed url reflect the selected filters on the issue dashboard page and project's issues page.
## Are there points in the code the reviewer needs to double check?
The solution isn't based on javascript and filters functionality but on the current url parameters. It didn't seem right especially when writing tests.
## Why was this MR needed?
After filtering the issues, users may press the feed button and see a different list of issues to what is shown on the HTML page
Closes#22947
See merge request !7259
Rodolfo Santos