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

[ewinslow]

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

Refs #14

[ewinslow]

@Elgg/team-elgg thoughts on this?

[brettp]

Like the idea, but I think it'd be better to use a select input for group selection. A newbie doesn't know which groups are available, and if we hand-pick the groups to display we can funnel people into some of the more popular (and usually more appropriate) groups.

[ewinslow]

We could just list the features groups then, maybe?

[brettp]

Yes, good idea.

[ewinslow]

OK I decided to use <datalist> for this. It doesn't work in all browsers (about 25% of our visits to the community, according to analytics) but I think is a good compromise that shows initial suggestions but also doesn't limit you to those if you know what group you're looking for. That number also will naturally go down over time as browsers are mostly auto-upgrading these days.

[ewinslow]

Pulling this in as no further objections in a couple weeks.