v3.14: missing token issuer URL sanitization

fts-rest-flask is not doing any sanitization of the token URL to ensure that it is ending with a /, despite it does it on the token providers retrieved from the DB. As a result, if the token issuer is not ending with a / it cannot be matched against one of the configured provider.

I'm happy to take care of this issue but cannot assign it to myself as I am not a project member (PR coming soon).