FIX: medium URL with @ incorrectly handled as mention

Also:

- remove unused code
- rename bbcode_ruler to bbcode.ruler
- add md.core.textPostProcess.ruler to apply at end of chain (excluding links)

plugins/discourse-details/assets/javascripts/lib/discourse-markdown/details.js.es6

@@ -25,6 +25,6 @@ export function setup(helper) {
   ]);
 
   helper.registerPlugin(md => {
-    md.block.bbcode_ruler.push('details', rule);
+    md.block.bbcode.ruler.push('details', rule);
   });
 }