Although /mf2/ posts can contain URLs, it wasn't up until this point
that we rendered them as HTML links.

This would hamper user experience for both human and computer readers
as it would remove some of the semanticness of the relevance of there
being a link there, as well as make it harder to follow the links
through.

To get this rendering, I've adapted [0] and [1] to produce a regex which
converts the plain URL to an HTML link

Additionally, convert any existing links that would break as part of
this change, as they're rendered next to punctuation that is valid in a
URL.

[0]: https://stackoverflow.com/a/29747837/2257038
[1]: https://stackoverflow.com/a/7109208/2257038