* Adds Runtime::Scenario as a place to record the properties of the
  Scenario we're interested in.

* Adds a positional argument to Test::Integration::Mattermost to specify
  Runtime::Scenario.mattermost

* Adds Page::Mattermost::{Main,Login} to model the mattermost pages we
  need to interact with.

* Adds qa/features/mattermost/login_spec to perform a login via oauth
  and ensure we are logged in.