Create sample end-to-end feature tests

[brettdh]

split from #15

Motivation

For some basic user workflows, we would like to be able to
simulate a user stepping through the app and assert that the
app's behavior matches expectations. This probably won't be
practical until we've made more progress implementing the app
as designed, but we can get started with some pieces that will help
testing and development in kind.

Plan of action (could be split off into sub-issues)

• Create a mock API server
  • Express seems like a good starting point
  • Devs should be able to run the server locally and point their
    local test app at the server rather than the "real" API server.
• Get calabash and cucumber up and running
  • Choose some simple workflow (e.g. login) and make a start-to-finish test description for it
• Get CircleCI running the simple test