Add twtxt support

[epif4nio]

The Twtxt protocol is a plain-text based microblogging protocol. This pull request adds a /@.txt path that generates a Twtxt-compatible feed.

This pull request has contributions from:

• @jhp0tter - original PR in the mastodon repo: ([draft] Add support for Twtxt protocol mastodon/mastodon#20745)
• @marado - convinced me to do this and tested it

CC/ @prologic

resolves #2745

[falsifian]

I noticed there's code here for dumping a varying number of twts, defaulting to the most recent 20.

If you want to do this in a way that's compatible with other twtxt implementations, have a look at the archive feeds extension. It allows you to serve a relatively small file but allows clients to look further back if they wish. At least, jenny and yarnd support it. It's probably more work than what you have here, but could improve interoperability with those clients.

Hope this is helpful. Just a random passer-by here.