Activity feed: Replace tabs with search bar

Problem to solve

In the current Activity feed (Activity dashboard), we display a two-level tab bar that allows filtering down for specific activities. This is a pattern we use on multiple pages and is not a good design. It is context-specific and unintuitive. Let's find a better solution.

Proposal

Proposal in https://gitlab.com/gitlab-org/gitlab-ce/issues/48321 by @cperessini:

The tab bars at the top of the page are replaced with a search bar, and the individual tabs are converted into filters (Type: Push events, Type: Merge events, etc.). This bar will also allow users to perform plain-text searches. Since search functionality is not yet available for this dashboard, we will keep the tabs in the first iteration.

Final design

What does success look like, and how can we measure that?

There's an opportunity to implement tracking events on the new search bar to learn about which filters are used most often.