Rename to Flags, tweak flag controls

- Display post "destructiveControls" in flag instead of custom buttons
- Make flags more versatile/extensible
- Delete associated flags when a post is deleted

Uninstall the Reports extension before installing.

extensions/flags/js/forum/Gulpfile.js

@@ -2,6 +2,6 @@ var gulp = require('flarum-gulp');
 
 gulp({
   modules: {
-    'reports': 'src/**/*.js'
+    'flags': 'src/**/*.js'
   }
 });