Replace links with HTML <a> tags when messages have a formatted_body and in all other cases

This makes (https) links show up as actual links when they're in a message with a `formatted_body`. For example, the pursuivant messages in any KDE channel (like #kirigami), or links which are in a reply to another message/image.

Also corrected a regex mistake in another place using the same link replacement regex.

Fix #331