* Whitelist allowed classes for federated statuses

Allowed classes are currently:

 - Any microformats class (h/p/u/dt/e-*)
 - the classes mention, hashtag, ellipses and invisible.

this last one is somewhat suspect, but Mastodon currently uses it to render hidden link text.

resolved #3790

* Fix code style

* Remove some arguments of Formatter.instance.format

* Improve spec for Formatter

Resolve #3270

so that when a reblog parse fails on another instance, it doesn't
look like a misattributed/stolen text

* Fix regressions from #2683

Properly format spoiler text HTML, while keeping old logic for blankness intact
Process hashtags and mentions in spoiler text
Format spoiler text for Atom
Change "show more" toggle into a button instead of anchor
Fix style regression on dropdowns for detailed statuses

* Fix lint issue

* Convert spoiler text to plaintext in desktop notifications

* fix character escaping in URL

* add tests

* put a comma after the last item

* add HTML escape test

Try to fix Travis CI setup

affected, replaced onClick handler with onMouseUp/Down to detect text
selection not trigger onClick handler then

to sign in page after sign up instead of root path which redirects to /about

as well as formatting of content