Added acceptance test for github theme installs
Added acceptance test for github theme installs

refs https://github.com/TryGhost/Team/issues/480

- testing happy path
  - mock github api request with redirect to zip-file download endpoint
  - theme is installed with a name of the github repo
  - response matches a theme upload response

TryGhost/Ghost,janvt/Ghost,TryGhost/Ghost,ErisDS/Ghost,janvt/Ghost,e10/Ghost,e10/Ghost,ErisDS/Ghost,ErisDS/Ghost,bitjson/Ghost,bitjson/Ghost,TryGhost/Ghost