This relaxes our very strict username rules to allow for some long asked for requests
- leading _ is now allowed
- . is allowed except for trailing char and confusing extensions like .gif .json
- dash (-) is now permitted
The userSearch() function, used for @mention autocomplete, returned an empty
list if the query string included non-latin characters or spaces. This removes
this restriction, so you can search users by any characters in their display
name, including spaces.