Fix index names in migrations

This can be reverted when we upgrade to Laravel 5.7.

extensions/likes/migrations/2018_06_27_100700_change_post_likes_add_foreign_keys.php

@@ -9,6 +9,7 @@
  * file that was distributed with this source code.
  */
 
+use Flarum\Database\Migration;
 use Illuminate\Database\Schema\Blueprint;
 use Illuminate\Database\Schema\Builder;
 
@@ -26,15 +27,19 @@ return [
             })
             ->delete();
 
-        $schema->table('post_likes', function (Blueprint $table) {
+        $schema->table('post_likes', function (Blueprint $table) use ($schema) {
             $table->foreign('post_id')->references('id')->on('posts')->onDelete('cascade');
             $table->foreign('user_id')->references('id')->on('users')->onDelete('cascade');
+
+            Migration::fixIndexNames($schema, $table);
         });
     },
 
     'down' => function (Builder $schema) {
-        $schema->table('post_likes', function (Blueprint $table) {
+        $schema->table('post_likes', function (Blueprint $table) use ($schema) {
             $table->dropForeign(['post_id', 'user_id']);
+
+            Migration::fixIndexNames($schema, $table);
         });
     }
 ];