Create and use MessageList widget
Our mailbox actions (inbox and outbox) were doing their own display of messages. This was causing issues with especially showmessage, which since the more rigourous nickname checks were added, no longer works as a mailbox subclass. I've taken the time to rip out the message listing code from MailboxAction and moved it to a MessageList widget. The different mailboxes now have their own subclasses that show the correct profile in the list.
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