Merge branch '24161-non-intuitive-buttons-for-import-sources-in-administrator-settings-enable-disable' into 'master'

Import sources buttons to checkboxes

## What does this MR do?

Swaps the `/application_settings` `Import Sources` buttons to checkboxes.

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

## Why was this MR needed?

## Screenshots (if relevant)

![2016-11-19_14.42.39](/uploads/5bf53ecb54765bd80ab275714208ee9a/2016-11-19_14.42.39.gif)

## Does this MR meet the acceptance criteria?

- [x] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] 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?


Closes #24161

See merge request !7598