Add a per-notifier ShouldNotify()

This way we are able to edit notification behavior per notifier.
This would be usefull to let some notifiers send notifications,
even when the state doesn't change, or with custom condition.

Signed-off-by: Thibault Chataigner <t.chataigner@criteo.com>

pkg/services/alerting/interfaces.go

@@ -15,7 +15,7 @@ type Notifier interface {
 	Notify(evalContext *EvalContext) error
 	GetType() string
 	NeedsImage() bool
-	PassesFilter(rule *Rule) bool
+	ShouldNotify(evalContext *EvalContext) bool
 
 	GetNotifierId() int64
 	GetIsDefault() bool