This introduces a new per-user preference called
"Auto-play animated GIFs", which is enabled by default. When a
user disables this setting, gifs in toots become click-to-play.

Previews of animated gifs were changed to display the video play
button so that users can distinguish them from regular images.

This setting also affects account avatars in the detailed account
view, which was changed to use the same hover-to-play mechanism
that is used for animated avatars in timelines.

Fixes #1652

* Put a useful message for new admins on /about/more

I totally failed to realize this file was just defaults.  I think
this message would be a good default for people like me.

* Revert default site description, expand comment

This will keep setup-related stuff from leaking into public views,
while still hopefully keeping over-eager admins from editing this
file unnecessarily before RTFMing.  (e.g., me)

with YAML-defined defaults. Add admin page for editing global settings. Add "site_description"
setting that would show as a paragraph on the frontpage