* Add missing fields group on preferences page

* Clean up settings/preferences controller

* Extract a UserSettingsDecorator

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

to only those from local users; Add link to "extended information" to
getting started in the UI; Add defaults for posting privacy; Change
how publish button looks depending on posting privacy chosen

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