Fix bug where pagination is still displayed despite all todos marked as done
## What does this MR do?
Changes the todo `mark all as done` button callback to remove pagination and add user friendly text `"You're all done!"` so that it is consistent with what the page should look like after refreshing
## Are there points in the code the reviewer needs to double check?
Shouldn't be
## Why was this MR needed?
Resolve existing UI inconsistency
## Screenshots (if relevant)
Before:
![8xDRiXB1TL](/uploads/479edbdedf8d468b4c5e502f36a0c19f/8xDRiXB1TL.gif)
After:
![q4XDxhujPX](/uploads/3566eb538e6ad5a2d32b78351b9bc47d/q4XDxhujPX.gif)
## Does this MR meet the acceptance criteria?
- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
- [x] Added for this feature/bug
- [x] All builds are passing
- [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 you do - 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#21048
See merge request !6113