This commit makes it possible to override settings during tests, by
switching capsulflask/__init__.py to a "create_app" pattern, and using
`dotenv_values` instead of `load_dotenv`.
The create_app() method returns a Flask app instance, to give
more control over when to initialise the app. This allows setting
environment variables in test files.
Then, use dotenv_values to override loaded .env variables with ones from
the environment, so that tests can set `POSTGRES_CONNECTION_PARAMETERS`
and `SPOKE_MODEL` (possibly others in future..).
Inital tests for the "landing" pages, and login / activation, are
included.
