Activity delivery logic should be in ActivitySend plugin

Problem/Motivation

The activity system uses ActivitySend plugins to deliver notifications through various channels (e-mail/webpush/other). The activity_send_push module however does all its logic in the hook_activity_insert implementation. It still calls the send plugin but it doesn't actually do anything.

Steps to reproduce

Proposed resolution

Move all the logic for sending the notification from the activity insert hook into the activity send plugin.

Remaining tasks

User interface changes

API changes

Data model changes