Minimal links fixes on social pages

Some links on social pages still refer to Mastodon and are confusing. The goal of this ticket is to implement very-low-effort temporary solutions. Explicitly allowable solutions include:

• hardcoding strings and links
• commenting out blocks

Using https://social.librem.one/@purism/102722652020959991 as an example:

• Librem Social logo. Stays the same.
• About. Link to https://librem.one
• Mobile apps. Remove.
• Log in. Stays the same.
• Sign up. Link to https://librem.one
• Entire footer. Remove. (All relevant links are in the instance description block.)

See also #12 (closed)