Only search the member list for nicknames that are actually visible

Closes #471 (closed)

Previous behaviour

Steps to reproduce:

1. Create a user with a nickname
2. Pick a preference that hides this nickname
3. Find the user through member search

New behaviour

Steps to validate that it works:

1. Create a user with a nickname
2. Pick a preference that hides this nickname
3. No longer find this user (but still find other users by nickname)