Add code-generation & linter for system notification templates

[dannykopping]

Ideally this code generation would produced a file called coderd/notifications/system_templates_generated.go:

// Workspace Events
var (
    TemplateWorkspaceDeleted = uuid.MustParse("f517da0b-cdc9-410f-ab89-a86107c420ed")
        ...
)

func AllSystemTemplates() []uuid.UUID {
  return {
    TemplateWorkspaceDeleted,
    ...
  }
}

// GetSystemTemplateByID returns the static contents of the requested system notification template.
func GetSystemTemplateByID(id uuid.UUID) database.NotificationTemplate {
  switch id {
    case TemplateWorkspaceDeleted:
      return database.NotificationTemplate {
        id: id,
        name: "Workspace Deleted",
        ...
      }
      ...
  }
}

NOTE: this must ONLY consider templates of kind=system.

We need this code-generation to solve a few problems:

1. Tests that currently reference the notification_templates table won't need to use a real pg db anymore, speeding up tests
2. AllSystemTemplates() will allow us to determine if we're missing any test cases
3. We could generate golden test files to validate that a template produces precisely identical HTML + plaintext outputs for given inputs

Over and above this, we'll need a linter to validate that this new file is not out-of-sync with the database after migrations have run.

[BrunoQuaresma]

Right now, the delivery methods are smtp and webhook, but they are typed as regular string values in the codersdk. I think it would be great to have the method value as enum types in the SDK to ensure more reliable type checking in the BE and FE. I see they are enums in the database layer, though.