Remove remaining "Mastodon" references

Smilodon's opt-in workflow is significantly different to Mastodon. To set user expectations accordingly, references to "Mastodon" should be removed. This task is mostly complete, but some stragglers remain.

These changes should be implemented in a white-labelling-friendly way, rather than replaced with hardcoded references to Librem One / Librem Social. Harcoded links to https://source.puri.sm/liberty/smilodon are ok, but using the name "Smilodon" is unnecessary (confusing for everyday users).

Post page

1. About / mobile links. These can be removed because all we need is...

2. Parameterize this so we can configure it to Back to Librem One with a link back to https://librem.one/accounts/profile/

3. Sidebar image. Already parameterized.

4. Post footer. A lot of this is specific to Mastodon or to a single-service domain, and we link to our policies and code in the sidebar. Here's our middleware footer for comparison:

Dashboard

5. Dashboard footer. Can we make this simpler too (see above)?

7. (7a) I think this is the last occurrence of toot :D

Profile editor

6. Could this be "Back to dashboard", or parameterized as "Back to Librem Social"?

Login

7. (7b) Can we replace this with our register / forgot password text and parameterized links? See below...

8. The favicon.