notifications: integrate webpush dispatcher into rest of notifications subsystem

[johnstcn]

coder/coder#17091 added a webpush package that supports sending notifications over WebPush.

This was not integrated into the rest of the notifications subsystem because its preferred mode of usage is more akin to UDP than the existing "TCP" reliable notification delivery system.

In order to integrate this properly, we'll need to do the following:

1. Add the ability to immediately dispatch a notification while skipping the enqueuing process. This should be a new code path and should not be configurable by end-users (i.e. "immediate dispatch" versus "retry dispatch" should be hard-coded by design)

2. Add the ability to dispatch a notification with an arbitrary message body. This should only be possible with arbitrary dispatch for now, as the current architecture for retryable dispatch requires a template to be stored in the database.

3. Add the webpush dispatcher as a "first-class" notification target.

Notes:

We'll have to be very careful about how we name things here so as not to conflate "webpush" notifications with the concept of a "notification" in coder/coder.

Tasks:

• Allow immediately* dispatching a notification.
• Add the ability to dispatch a notification with an arbitrary message body
• Add the webpush notifier as a "first-class" notification target

* or as close to immediately as possible

[johnstcn]

Alternative implementation:

• Add LISTEN/NOTIFY support for notification dispatch to reduce time-to-dispatch for notifications. This is already a TODO called out in existing code.
• Leverage existing 7-day cutoff for deleting old notification messages.

This should enable us to keep a single code path for both kinds of notifications.