Event or hook not fired on Hub Subscribe

Problem/Motivation

Modules should be able to hook into the creation of a subscription to generate notifications or other actions. Normally with most entities this can be done with hook_ENTITY_TYPE_insert

Steps to reproduce

Creating a hook_subscription_insert doesn't do anything, as Subscriptions don't implement entity API

Proposed resolution

We can:

• Refactor Subscription to use entity API (probably a bad idea)
• Create a new event dispatcher to allow eventsubscribers to act on new subscription on the hub site