improve video button visibililty (issue #713)

Add social.targaryen.house to instances list

Add mastodon.xyz to instances list

Put line breaks between API methods in docs

Clarify post privacy warning

Mastodon is not an alternative, it is a solution

This month I traveled to Cambridge, Massachusetts with my friends and attended LibrePlanet 2017. At the talk *Technology for direct actions*, Andrew Seeder pointed out that we often call free software an "alternative" to what exists. But an alternative is something that is mostly similar, and differentiated by preference rather than need. For instance, a trackball is an alternative to a mouse, but a jail is not an alternative to a bedroom. Andrew suggested that we start looking at free software as "solutions" rather than alternatives. So I propose boldly proclaiming that Mastodon is not an alternative to Twitter, but rather a solution to it.

Needs to be a separate task with no locking