Add 'Cancel all imports' button and confirmation dialog when importing projects from GitHub
<!--IssueSummary start-->
<details>
<summary>
Everyone can contribute. [Help move this issue forward](https://handbook.gitlab.com/handbook/marketing/developer-relations/contributor-success/community-contributors-workflows/#contributor-links) while earning points, leveling up and collecting rewards.
</summary>
- [Work on this issue](https://contributors.gitlab.com/manage-issue?action=work&projectId=278964&issueIid=385874)
- [Close this issue](https://contributors.gitlab.com/manage-issue?action=close&projectId=278964&issueIid=385874)
</details>
<!--IssueSummary end-->
The following discussion from !103934 should be addressed:
- [ ] @xanf started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/103934#note_1209118861): (+5 comments)
> @gdoyle could you please give us a hand with ~UX review?
**non-blocking:** The [design](https://gitlab.com/gitlab-org/gitlab/-/issues/247325) included a Cancel imports button when you bulk import many projects. I didn't see that appear on the screen. Will this be done in a follow-up? 
## Proposal
- Add `Cancel all` button based on this [design](https://gitlab.com/gitlab-org/gitlab/-/issues/247325).
- Add a confirmation modal when taking this action with the following text :point_down:
> #### Cancel remaining imports?
>
> **6 repositories** are still importing. Imported files will be kept. You can reimport these repositories again later.
>
> `Continue importing` {-Cancel remaining imports-}
~"Technical Writing" review: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/103934#note_1211554229
issue