Each time a deprecated webhook is hit, a notification is
created (without duplicating it) to be sent.

* Updated copy on webhooks

* Drop "deprecated webhook endpoint" copy, this is core team nomenclature, not user
nomenclature.
* Add small amount of docs to point to

* Update docs and point to docs in notification message

* Add year

…points (#5083)

* Updated copy on webhooks

* Drop "deprecated webhook endpoint" copy, this is core team nomenclature, not user
nomenclature.
* Add small amount of docs to point to

* Update docs and point to docs in notification message

* Split up deprecated view notification to GitHub and other webhook endpoints

This sets a date for deprecated of these endpoints as Mar 1st 2019. Too
soon?

* Reduce complexity and drop decorator pattern for Notification
  classmethod pattern used in other notifications
* Add notifications for non-GitHub incoming webhooks
* Add docs as well

* More renaming and slight refactor

Found out 2x messages are being generated, so this stops the automated
mechanism for triggering these messages.

* Update dates

* Also update docs

* Typo on date

* Back out some more of the changes to notifications to make them operable without automation

* Add admin method for notification

* Add admin filter for project features