Text for link to other instances

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

Federate spoiler_text using warning attribute on <content /> instead of a <category term="spoiler" />
Clean up schema file from accidental development migrations

application website validation, don't link to app website if website isn't set,
also comment out animated boost icon from #464 until it's consistent with non-animated version

A couple of places were using "reblog" rather than "boost" - this updates them to match the web UI

Better initial state for unlisted/nsfw toggles

fix #361 with rich OEmbed display via iframe, fix #237 by hiding sensitive
content behind a spoiler on public pages