Forbid @ in usernames

[xaur]

I have not analyzed all pros/cons but intuitively I'm pretty sure it's better to forbid @.

At the very least, to support @mentions in the future without confusing @@username cases.

Might be part of #1479

[xaur]

Expanding this a bit, maybe also forbid leading underscore _. Not sure about trailing _ yet. Should be ok in the middle. Reason - URLs like this mess up our Markdown far too often in documents and our Matrix chats (sorry Checkmate):

https://twitter.com/_Checkmatey_

Fewer tools for impostors is good, too.

This is a handy recap of Twitter username policy to borrow from. I guess some restrictions are already implemented in Pi but just in case:

• 15 characters max
• only alphanumeric and underscore
• interesting bit, some words are reserved like Admin or Twitter (admin or politeia in our case, maybe more)

[lukebp]

This is already documented and being taken care of. We don't need a separate issue for it.

#1479 (comment)