This builds on Rich's pull request https://github.com/joindin/joindin-api/pull/25

It takes account of some of Akrabat's changes that happened in between.  We 
store a string identifier in the "source" column of the comments tables, in line
with how this has historically been done on the old site.  m.joind.in and any
other non-standard oauth clients will show as "joind.in"