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.

tests/integration/api/posts/ListTest.php

@@ -42,7 +42,7 @@ class ListTests extends TestCase
 
     private function forbidGuestsFromSeeingForum()
     {
-        $this->database()->table('group_permission')->where('permission', 'viewDiscussions')->where('group_id', 2)->delete();
+        $this->database()->table('group_permission')->where('permission', 'viewForum')->where('group_id', 2)->delete();
     }
 
     /**