Transform @handle mentions into clickable links
Known bugs:
-when editing a "linked" text box (caption, etc), the entire HTML tag for the link shows up. This is mostly a fetlife bug, but I will attempt to fix it on my end.
-@OP or @me get directed to the literal profiles instead to the original poster or the person who made the comment/post
-Sometimes pictures down-size slightly with the extension enabled. I believe a new HTML element is getting created at the end of the caption, which causes the page to re-arrange content