Refresh timelines when streaming API reconnects in the UI

Expose ActivityStreams 2.0 representation of accounts

Add "clear notifications" button, exclude posts from people who have blocked *you* from public/hashtag timelines

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

Try to fix Travis CI setup

warning mentions preview into area where actual content would be

overshadow requirement for remote-lookup

untested, my vagrant is bugged again, submitted at gargron's request

Make the streaming API also handle websockets (because trying to get the browser EventSource interface to
work flawlessly was a nightmare). WARNING: This commit makes the web UI connect to the streaming API instead
of ActionCable like before. This means that if you are upgrading, you should set that up beforehand.