Tidier and complete fixes on social pages
This is a follow up to https://source.puri.sm/liberty/smilodon/issues/14
Rather than minimal fixes we should ideally:
- parameterize host-specific values
- reinstate a high-value low-clutter footer
- the Mastodon footer has too much
- source code link should be moved to footer
- this will need some design discussion
We should also:
- remove all references to "Mastodon" (replace with "Social" or "ActivityPub" as appropriate, or remove unused strings entirely) - see https://source.puri.sm/liberty/smilodon/merge_requests/5 for a first attempt