Fix processing of Web Push and streaming notifications

Original issue description: this has been plaguing me for a while. When I mute a thread I still receive notifications for it on my apps (e.g., Mona on iOS).

Identified problem: federated activities generate notifications without any filters and send them (SideEffects.handle_after_transaction)

Updated status:

• Notification.send/1 now exists
• Notifications are no longer sent as soon as they are generated (unless passed the flag do_send: false; this functionality has been removed)
• Federated and local activities which come through the common pipeline always accumulate the notifications for sending at the end of the pipeline (SideEffects.handle_after_transaction)
• local Create activities do not go through the pipeline yet, but CommonAPI.post -> ... -> ActivityPub.notify_and_stream now collects the notifications and sends them with Notification.send/1
• We no longer generate activities from users/threads that are blocked/muted. They were previously generated but not sent. This wastes database space for content you didn't want in the first place.

There are a couple test failures but I believe the tests are wrong