Rich editor misses usernames with umlauts

[mcwumbly (Dave McClure)]

Thanks for the report. We’ll take a look!

I was able to repro after configuring the following settings:

1. configuring these settings to allow some unicode characters in usernames
   • unicode_usernames
   • allowed_unicode_username_characters (e.g. [äöüßÄÖÜẞ])
2. create a user using one of those chars in their username
3. try to mention them in a post using the new rich text editor

We should double check that mentions for group names w/ unicode characters work too when we fix this.

Related: Unicode usernames and group names