Introduce new flask based python backend

This supports most functionality. Tested the following functionality:
* Create account
* Delete account
* Create a card
* Download offline copy (couldn't log in) - needs work
* Change passphrase
* One time password creation and use

backend/flask/src/clipperz.py

@@ -0,0 +1,9 @@
+from clipperz import app, db
+
+
+def main():
+    db.create_all()
+    app.run(debug=True)
+
+if __name__ == "__main__":
+    main()