Update config handling and check token issuer
Update config handling and check token issuer

Use setdefault to set default config options instead of a
try-catch with every option.
Also implement checking of token issuers.

Signed-off-by: Patrick Uiterwijk <cbb7353e6d953ef360baf960c122346276c6e320@puiterwijk.org>

SteelPangolin/flask-oidc,puiterwijk/flask-oidc