Tweak release page source links
This makes it more clear that the source downloads are snapshots
and that the latest source code is available elsewhere.
This also switches to an icon that is more easily visible in smaller fonts.
Depends on D7615
Navigate to http://127.0.0.1:4000/releases/
Reviewers: #bitcoin_abc, Fabien
Reviewed By: #bitcoin_abc, Fabien
Differential Revision: https://reviews.bitcoinabc.org/D7619