[feature] Format links correctly in statuses etc

[tsmethurst]

We should allow users to post links and format them nicely so they're clickable HREFs.

To determine if part of a post is a link, we can use something like this (if we don't feel like rolling our own regular expressions):

https://github.com/mvdan/xurls

We need to parse and replace links with HREF's before extracting hashtags from a status, to make sure we don't include link fragments as hashtags.