Harmonize H. favicon

## Feature Request Form
### Problem you are trying to address with this feature
Our favicon for Hypothes.is uses the older outlined logo, vs the newer solid one.

### Your solution
Perhaps with the release of activity pages, we might update it. The tab at left in the image below is the favicon that shows with zendesk, which we might consider using.


The issue provides only a screenshot and a general request to replace the older outlined favicon with the newer solid logo; it names no repository files, tests, or implementation entry point. First confirm the intended logo and where the current favicon is defined with maintainers. Done means the agreed Hypothes.is favicon is used consistently.

Written by the indexing model from the issue text.