Tweak release page source links

Summary:
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

Test Plan:
make serve
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