Added missing .bp-suggestions for @mentions in BP 2.1

Looks as progress like @-mentions were forgotten to get ported back to good ol’ bp-default. In bp-activity/js/mentions.js you’ll find on what elements @-mentions should work:

```
// Activity/reply, post comments, dashboard post 'text' editor.
$( '.bp-suggestions, #comments form textarea, .wp-editor-area' ).bp_mentions( users );
```

The selector class 'bp-suggestions' is currently missing in bp-default.