Tag users as favourites

**Is your feature request related to a problem? Please describe.**
No

**Describe the solution you'd like**
An easily accessible way to tag a user as a favourite and display a list of favourite users.

**Describe alternatives you've considered**
Browser bookmarks linking to each individual user also work, but scale poorly if there are more than a handful users to keep track of.

**Additional context**
We often test features in production by toggling feature flags for our own users, and it would be nice to have a shortcut to a list of favourite users instead of looking up the user ID.

gz#442