SubscriptionRequest: Add block/report features
This adds two new options to 'Deny':
- Block User: will block the JID using XEP-0191: Blocking Command
- Report as Spam: will block the JID using XEP-0191: Blocking Command plus report the JID as spam using XEP-0377 Spam Reporting
Note that I removed the usage of XEP-0016: Privacy Lists in consequence of #10161 (closed).
I also took the opportunity to tweak the SubscriptionRequst window a bit.
This MR depends on python-nbxmpp!47 (merged) and fixes #10115 (closed).