feature(discussion): make it much easier for guests to post new topics

 * "Add discussion topic" always visible, even when logged out.
 * Group can be selected via autocomplete
 * Auto-join to selected group if possible and not already a member

Refs #14
feature(discussion): use datalist to recommend featured groups
Merge pull request #28 from Elgg/discussion-14

feature(discussion): make it much easier for guests to post new topics