Fixes issues with e.g. foo@hamlet.com (where hamlet is a user)
The \b was ineffective because @ made it always on a word boundary.
Instead, use the negative lookbehind trick from the URL regex.
(imported from commit fdca9bd686e4f8747e67b412cba1fa7c5c9391aa)
* This makes bugdown.convert take a `message` parameter. Properties
for parsed mentions are added to the message object by the `Pattern`
for use in do_send_messages.
* Refactor repeated markdown rendering code into `Message` model methods.
(imported from commit 4f0ed5570104c0210f984b6de21e9048e2b53fa0)
