poll optimizations: skip existing responses and activities without new responses
source.AS_CLASS in dashboard.html doesn't work *for Twitter only*. work around.

so weird, no clue why twitter alone fails like this.
drop poll back to 15m for now
drop log level for webmention errors to warning
permashortcitations are only at the end of posts
fix bad favicon path
TODO: maybe interpret oauth2client.AccessTokenRefreshError as revoked. maybe not though.
drop poll task if source is disabled
add facebook revoke and error handling
use "account" in user visible text, not "source"