Fix group/permission seeding

Updating the Migration::addPermission helper table name means we need
to move the seed migration to after the table rename migration. We also
add a sanity check for each permission's group since the foreign key
will fail if the group doesn't exist. Of course, the only way to make
sure groups are seeded before permissions is to move them into another
migration.

migrations/2018_07_16_003952_allow_hide_posts_for_moderators.php

@@ -1,17 +0,0 @@
-<?php
-
-/*
- * This file is part of Flarum.
- *
- * (c) Toby Zerner <toby.zerner@gmail.com>
- *
- * For the full copyright and license information, please view the LICENSE
- * file that was distributed with this source code.
- */
-
-use Flarum\Database\Migration;
-use Flarum\Group\Group;
-
-return Migration::addPermissions([
-    'discussion.hidePosts' => Group::MODERATOR_ID
-]);

migrations/2018_07_21_000000_seed_default_groups.php

@@ -0,0 +1,38 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * (c) Toby Zerner <toby.zerner@gmail.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+use Flarum\Group\Group;
+use Illuminate\Database\Schema\Builder;
+
+return [
+    'up' => function (Builder $schema) {
+        $db = $schema->getConnection();
+
+        $groups = [
+            [Group::ADMINISTRATOR_ID, 'Admin', 'Admins', '#B72A2A', 'fas fa-wrench'],
+            [Group::GUEST_ID, 'Guest', 'Guests', null, null],
+            [Group::MEMBER_ID, 'Member', 'Members', null, null],
+            [Group::MODERATOR_ID, 'Mod', 'Mods', '#80349E', 'fas fa-bolt']
+        ];
+
+        foreach ($groups as $group) {
+            if ($db->table('groups')->where('id', $group[0])->exists()) {
+                continue;
+            }
+
+            $db->table('groups')->insert(array_combine(['id', 'name_singular', 'name_plural', 'color', 'icon'], $group));
+        }
+    },
+
+    'down' => function (Builder $schema) {
+        // do nothing so as to preserve user data
+    }
+];