Overhaul permissions

Get rid of Permissible - too complex and inefficient. Replace with:
- a “Locked” trait which works similarly but only evaluates logic on
hydrated models.
- a “VisibleScope” trait which also works similarly but only scopes
queries

This is all we need, Permissible is overkill. There is only one
instance where we have to duplicate some logic
(Discussion::scopeVisiblePosts and Post::allow(‘view’, …)) but it’s
barely anything.

Haven’t decoupled for now, we can definitely look at doing that later.

Permissions table seeder slightly updated.

Also did a bit of a query audit, there’s still a lot to be done but
it’s much better than it was. Some relatively low-hanging fruit
detailed in EloquentPostRepository.

src/Core/Support/VisibleScope.php

@@ -0,0 +1,20 @@
+<?php namespace Flarum\Core\Support;
+
+use Flarum\Core\Models\User;
+
+trait VisibleScope
+{
+    protected static $visibleScopes = [];
+
+    public static function scopeVisible($scope)
+    {
+        static::$visibleScopes[] = $scope;
+    }
+
+    public function scopeWhereVisibleTo($query, User $user)
+    {
+        foreach (static::$visibleScopes as $scope) {
+            $scope($query, $user);
+        }
+    }
+}