Implement support for private messages

[soapdog]

This has been outlined originally in the new funding proposal of 2021: %Y2iyBckEhcWvxX6a1+9ELeP6wFYD80QNdhvR50V1C7E=.sha256.

Add private message support

One crucial missing piece in Patchfox is private messaging. I don’t mean the new private groups, I mean the old-style common private messages. At the moment, you can kinda read them in your timeline, but you can’t write them.

I want to make private messaging a first-class feature by integrating it as a Patchfox mini-app called Inbox.