Rename viewDiscussions => viewForum, viewUserList => searchUsers (#2854)

This naming is clearer as to the intended effect. Changes include:

- A migration to rename all permissions
- Updating the seed migration to use the original naming from the start
- Replacing usage of the old names with new names in code
- Throwing warnings when the old names are used.

src/Api/Controller/ListUsersController.php

@@ -75,7 +75,7 @@ class ListUsersController extends AbstractListController
     {
         $actor = RequestUtil::getActor($request);
 
-        $actor->assertCan('viewUserList');
+        $actor->assertCan('searchUsers');
 
         if (! $actor->hasPermission('user.viewLastSeenAt')) {
             // If a user cannot see everyone's last online date, we prevent them from sorting by it