MM-25394 session expired push notifications (#14732)

* new job type created that checks for expired mobile sessions and pushes notifications.

* only send session expired notifications if ExtendSessionLengthWithActivity is enabled.

* includes schema change:  field added to Sessions table

app/app_iface.go

@@ -226,6 +226,8 @@ type AppIface interface {
 	NewWebConn(ws *websocket.Conn, session model.Session, t goi18n.TranslateFunc, locale string) *WebConn
 	// NewWebHub creates a new Hub.
 	NewWebHub() *Hub
+	// NotifySessionsExpired is called periodically from the job server to notify any mobile sessions that have expired.
+	NotifySessionsExpired() *model.AppError
 	// OverrideIconURLIfEmoji changes the post icon override URL prop, if it has an emoji icon,
 	// so that it points to the URL (relative) of the emoji - static if emoji is default, /api if custom.
 	OverrideIconURLIfEmoji(post *model.Post)