Remove wiki content and rename this repo

We've moved to using MediaWiki at wiki.social.coop. The only page that we need this webapp to deliver now is the /registration-form.html page to display the registration form for Mastodon. The meet.social.coop registration form can be removed since that is sadly no more.

• Remove all wiki content and routes except for routes related to the Mastodon registration form
• Remove the menu which points at different parts of the wiki (or consider repurposing to point at our Mastodon, new Wiki and Loomio).
• Rename the git repository to join.social.coop
• Remove the commit hook from General repository which pings a webhook in this app to update the wiki content.
• Simplify the join.social.coop Ansible role by removing config that is no longer needed.
• Deploy 🚀
• Party 🎉