Register-dependant user search in Private messages

When searching for the receiver of the private message in the "To" field, the register has to match the register of receiver's nickname set during registration

Reproduction steps:

1. Login to the forum with valid credentials
2. Open "Messages"
3. Click "Compose"
4. Put the nickname of the user to whom the message shall be addressed in the "To" field

Test data:

• username: Tauretary

ER: no error messages, user found

AR: error message "User not found" and dropdown list with the actual nickname ("tauretary") appear

Build version:  3.13.3180.ed3a86a (no matches in JIRA)