Expand accepted characters for username syntax

## Feature Request Form
### Problem you are trying to address with this feature
Currently, characters such as é are not accepted in usernames - as we expand our user base, people will want to use characters from their real names (and from various words in their native languages) as part of their usernames.

### Your solution
Consider expanding the charset to accept all Unicode characters. This won't solve the problem for everyone, since Unicode has its own issues re: inclusivity and diversity, but it would be a good start.

See customer support ticket: https://hypothesis.zendesk.com/agent/tickets/1978