Fix index names in migrations

This can be reverted when we upgrade to Laravel 5.7.

migrations/2018_09_15_043621_add_posts_indices.php

@@ -9,23 +9,28 @@
  * file that was distributed with this source code.
  */
 
+use Flarum\Database\Migration;
 use Illuminate\Database\Schema\Blueprint;
 use Illuminate\Database\Schema\Builder;
 
 return [
     'up' => function (Builder $schema) {
-        $schema->table('posts', function (Blueprint $table) {
+        $schema->table('posts', function (Blueprint $table) use ($schema) {
             $table->index(['discussion_id', 'number']);
             $table->index(['discussion_id', 'created_at']);
             $table->index(['user_id', 'created_at']);
+
+            Migration::fixIndexNames($schema, $table);
         });
     },
 
     'down' => function (Builder $schema) {
-        $schema->table('posts', function (Blueprint $table) {
+        $schema->table('posts', function (Blueprint $table) use ($schema) {
             $table->dropIndex(['discussion_id', 'number']);
             $table->dropIndex(['discussion_id', 'created_at']);
             $table->dropIndex(['user_id', 'created_at']);
+
+            Migration::fixIndexNames($schema, $table);
         });
     }
 ];