* Add mask-icon (#1572)

* use updated svg logo for mask-icon

used from nginx when Rails fails.

This allows other platforms such as Windows, macOS and iOS to use their system fonts rather than downloading a copy of Roboto. It also makes the app feel a little closer to native on those platforms!

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

Do not autoplay videos, display play button instead. Use expiring links when using S3. Do not keep originals
for avatars/headers, resize avatars down to 120x120 instead of 300x300. Set cache headers on S3 stuff, also
make it private (aka only accessible via expiring links to prevent hotlinking)

See: https://dequeuniversity.com/rules/axe/2.1/html-has-lang

registration API