Allow overriding url_prefix when used as a blueprint on app.register_blueprint

[Ambro17]

It currently breaks redirection as slackify never notices of such event.

Create failing test
Fix it
Check test passes

[Ambro17]

Now that we added support for dependency injection with a custom Blueprint, this fix should be easier. Just add some logic in the overriden register method to detect if the app has registered the blueprint under a different prefix and update it