Basic testing using flask-testing

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.

capsulflask/tests_base.py

@@ -0,0 +1,25 @@
+import os
+from nanoid import generate
+
+from flask_testing import TestCase
+
+from capsulflask import create_app
+
+class BaseTestCase(TestCase):
+    def create_app(self):
+        # Use default connection paramaters
+        os.environ['POSTGRES_CONNECTION_PARAMETERS'] = "host=localhost port=5432 user=postgres password=dev dbname=capsulflask_test"
+        os.environ['TESTING'] = '1'
+        os.environ['SPOKE_MODEL'] = 'mock'
+        return create_app()
+
+    def setUp(self):
+        pass
+
+    def tearDown(self):
+        pass
+
+    def _login(self, user_email):
+        with self.client.session_transaction() as session:
+            session['account'] = user_email
+            session['csrf-token'] = generate()