Use https scheme for OAuth redirects

[DeD1rk]

Using https instead of nu.thalia as scheme for OAuth redirects is more secure, since both iOS and Android require a web server to declare that they some app is allowed to listen to its https redirects.

For #239, we have to do this as TOSTI doesn't allow non-https redirects, but for concrexit it also seems like a good idea.

See https://developer.android.com/training/app-links/verify-site-associations

[DeD1rk]

This is actually not needed for TOSTI, although it still would be nice.

[DeD1rk]

Seems like this will be supported by flutter_web_auth2 soon.