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:
![4OzkoQVJYc](/uploads/fd2a7fdbde2159e875482ec7b828fe60/4OzkoQVJYc.gif)

After:
![E0lj8UhEUU](/uploads/7eb5155861eb79d72957de04c9f172c9/E0lj8UhEUU.gif)

## 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