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

Filters out hidden stream entries from Atom feed
Blocks now generate hidden stream entries, can be used to federate blocks
Private statuses cannot be reblogged (generates generic 422 error for now)
POST /api/v1/statuses now takes visibility=(public|unlisted|private) param instead of unlisted boolean
Statuses JSON now contains visibility=(public|unlisted|private) field

removing web app capability from non-webapp pages

Fix #233, fix #268

It can take as much as 0.5s if not longer to complete

Mostly resolves #8, though attachments are still not displayed in public view

DistributionWorker. That isn't the heaviest service, yet, but gotta start
somewhere

fetch webfinger information of mentioned accounts