Include User Name in Mention Autocomplete in addition to Full Name

The 'Mention' autocomplete seems to only work on Full Name. In the other hand, 'Share Page' autocomplete works on both Full Name and Username.

It'd be nice if the 'Mention' autocomplete can also work on User Name since there are some users that depends on username rather than full name. If this is a bit too much for 'mention', perhaps providing a switch option whether to use username vs. fullname vs. the both for its autocomplete would help.

Steps to reproduce:

1. Create a new user:
   Username: username
   Full Name: Full Name
2. Wait until it gets indexed correctly
3. Try the difference in 'Share Page' autocomplete vs. 'Mention' autocomplete.