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/Discussion/Access/ScopeDiscussionVisibility.php

@@ -20,7 +20,7 @@ class ScopeDiscussionVisibility
      */
     public function __invoke(User $actor, $query)
     {
-        if ($actor->cannot('viewDiscussions')) {
+        if ($actor->cannot('viewForum')) {
             $query->whereRaw('FALSE');
 
             return;