Fix inconsistent highlighting of already selected activity nav-links
## What does this MR do?
* Remove edge case where user could deselect an activity nav-link (which seems to be returning all the events)
* Explicitly add an `All` tab to return all the events
## Are there points in the code the reviewer needs to double check?
Shouldn't be
## Why was this MR needed?
Resolves existing UI inconsistency
## Screenshots (if relevant)
Before:
After:
## Does this MR meet the acceptance criteria?
- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
- [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#21631
* Closes#21452
See merge request !6091
a625757cb4