Added support for 2 letter forum usernames

[Byte]

Added support for 2 letter forum usernames.

Previously if you had a 2 letter username:

• People could not send you a PM
  
• People could not tag you. Ie if your username was GO then @go would not tag you
  

This is now fixed.

For documentation:
Added changes in this commit: https://github.com/mybb/mybb/pull/2195/commits
Also changed template in: private_send_autocomplete and memberlist and accountswitcher_author_admin and search


Another change is there is now no maximum PM limit (previously 3 max per PM, now unlimited).