Abstracted user avatar fetching away from gravatar

Still uses gravatar as a default.
Updated URL placeholders to follow LDAP format.
Potential breaking config change: `GRAVATAR=false` replaced by `AVATAR_URL=false`
Builds upon #1111

app/Http/Controllers/UserController.php

@@ -92,7 +92,7 @@ class UserController extends Controller
             $user->roles()->sync($roles);
         }
 
-        $this->userRepo->downloadGravatarToUserAvatar($user);
+        $this->userRepo->downloadAndAssignUserAvatar($user);
 
         return redirect('/settings/users');
     }