configure and check request.trusted_hosts

[davidism]

Add TRUSTED_HOSTS config. In Flask.create_url_adapter, which is called when creating a RequestContext, set request.trusted_hosts to the config. Call get_host with the config, and assign it to request.host to pre-cache the property.

I considered making flask.Request.trusted_hosts a property, so that setting it would trigger validation again automatically. I'm not confident this difference in behavior from Werkzeug, where only accessing request.host triggers validation, would be good. If it's a good idea, it can be done in Werkzeug.

fixes #5636