Add new default "Your activity" tab to the Activity dashboard

What does this MR do and why?

This adds a new default tab to the /dashboard/activity page. Until now, its default tab was "Your projects", showing all events in all projects the user is a member of.

This gets really noisy really fast and since this page is now part of the Your work sidebar (#384342 - closed), it makes more sense to scope the default Activity view to show only "Your activity", making it easier to find old comments, recently edited issues etc.

Implements #389997 (closed).

Screenshots or screen recordings

How to set up and validate locally

• Go to http://gdk.test:3000/dashboard/activity
• The first and default tab should show only Your activity.
• The now second tab shows your projects' activity.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.