Notify Wiki owners by email when users [request to] join their wiki

Steps to reproduce:

1. Login with an Admin
2. Create a global user (U1)
3. Create 2 subwikis: one Open (S1) and one where "Any global user can request to join but an admin must accept" (S2)
4. Login with U1 and:
   • Join S1
   • Request to Join S2
5. Login with the Admin from step 1

You don't have any notification that an user has joined S1 nor that the same user has requested to join S2. You have to manually go to each subwiki's Administration -> Users section to check if there's any new user or candidate.