Remove mozilla.social links and invite page

[craigcook]

One-line summary

Removes links to mozilla.social from the global footer and removes the invitation request page.

There is still a link to mozilla.social on the Nightly WNP but I'll talk to Pascal about how he wants to handle that, if he'll migrate that account to another instance or shut it down. We can update that page in a follow-up PR.

This also leaves the content policy page (/about/governance/policies/social-content-policies/) since I think we need to keep that up as long as mozilla.social is active. Once the instance actually closes down (planned for December) we'll be able to remove that policy.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.96%. Comparing base (1866841) to head (54741fd).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15234      +/-   ##
==========================================
+ Coverage   77.90%   77.96%   +0.05%     
==========================================
  Files         163      163              
  Lines        8483     8472      -11     
==========================================
- Hits         6609     6605       -4     
+ Misses       1874     1867       -7     

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[alexgibson]

r+wc 🌮

[janbrasna]

"[…] we have no intention to abandon our presence on Mastodon, we are just closing our own Mastodon instance. We will migrate our firefoxnightly account probably early December and will just update the url on the pages where it is mentioned."

— Originally posted by @pascalchevrel in #15188 (comment)