This works slightly better on iOS and on Android it has a keep alive,
which will prevent our app from being killed. Additionally, this way
there is less for me to maintain, which is always nicer.
The API for flutter_web_auth is also much simpler.
This also inolves some custom logic for parsing the Query Parameters
from the GitLab callback, as it doesn't seem to be a proper URI. Not
sure what is going on with Gitlab.
