Do not use the mail icon for new messages

For now messages, you currently use this icon:

This is bad as this is an icon for mail (apps) and is used by some popular ones, e.g. K9 mail. As Briar is not a mail app, you should not use this, otherwise users get confused, because they think they have a new mail, while they actually have a new Briar message.

Better use the icon (or a variation) of Briar itself. This makes clear that there are new Briar messages.