Add notifiers logic for resource threshold reached

[defelmnq]

⚠ subissue of coder #15746

In order for the OOM and OOD notifications to work - we need to change a bit the notifiers :

Create a new migration to add the template for the new events. It is a simple DB migration.
Modify both the FE and BE to add a new event with the preferences.
It should be done in the admin side to let admins choose if the notifications should be sent through webhooks or email
It should be done in the user side to let users turn it on or off.
⚠ This component is the last one that should be merged - once merged the connection between the processing part and the notifiers can be done.