ogcp: set default SameSite attribute in session cookie

Set the SameSite attribute to 'Lax' if not defined.

The SameSite attribute is a cross-site tracking prevention measure
and defines when cookies are sent within the HTTP headers and
the cases when they are not sent.

The 'Lax' configuration has the following features:
- Cookies are sent with same domain requests.
- Cookies are sent when the user navigates to your site by clicking
  a link or submitting a form from another site.
- Not sent with requests made from other sites such as embeded content.

Prevent warning from browsers such as Firefox from complaining
about undefined SameSite value.

ogcp/__init__.py

@@ -16,6 +16,10 @@ ogcp_cfg_path = 'cfg/ogcp.json'
 
 app = Flask(__name__)
 app.config.from_json(ogcp_cfg_path)
+
+if 'SESSION_COOKIE_SAMESITE' not in app.config or not app.config['SESSION_COOKIE_SAMESITE']:
+    app.config['SESSION_COOKIE_SAMESITE'] = 'Lax'
+
 app.secret_key = urandom(16)
 
 class NoScopeStatus(logging.Filter):