Consider allowing fully-custom notification IDs

Right now notifications always use their object's ID in their IDs. However, sometimes we might need them less unique. E.g. a test subscription might chose to only send one notification per hour, regardless of which object was notified about.

Consider letting subscriptions pick whatever ID they want for their notifications, but making sure they don't step on each other's toes.

Start by tracing how notification IDs are generated from object IDs and how subscriptions create notifications. Define the desired custom-ID and collision behavior, including the hourly test-subscription example, before identifying the relevant tests. Done means subscriptions can choose notification IDs without interfering with one another.

Written by the indexing model from the issue text.