Change as part of https://github.com/coder/coder/pull/16071
It has been decided that we want to be able to have some notification
templates be disabled _by default_
https://github.com/coder/coder/pull/16071#issuecomment-2580757061.
This adds a new column (`enabled_by_default`) to
`notification_templates` that defaults to `TRUE`. It also modifies the
`inhibit_enqueue_if_disabled` function to reject notifications for
templates that have `enabled_by_default = FALSE` with the user not
explicitly enabling it.
