Cannot focus lozenges using keyboard

**Steps to reproduce**

1. Using the keyboard, enter "tag:term" in the search input in the navbar
2. Attempt to delete the inserted lozenge using the keyboard

**Expected** - There is a clear and obvious way (eg. by pressing tab/backtab) to focus the lozenge and delete it and when the lozenge has keyboard focus, this is clearly visible
**Actual** - Keyboard focus skips over dynamically inserted lozenges

Related to https://github.com/hypothesis/h/issues/3991 , aside from supporting backspace to delete search terms, there are a couple of fundamental issues with keyboard accessibility of the search input:

1. The keyboard cannot be used to focus client-rendered lozenges
2. Although a server-rendered lozenge can be focused using the keyboard and removed by pressing the Enter key, there is no visual indication when the lozenge is focused