PLT-2664 Split out push notifications from email notifications (#3049)

* Split out push and email notification logic, always send push notifications on mention

* Add user setting to control push notifications

* Minor changeto simplify code

model/user.go

@@ -191,6 +191,7 @@ func (u *User) PreUpdate() {
 func (u *User) SetDefaultNotifications() {
 	u.NotifyProps = make(map[string]string)
 	u.NotifyProps["email"] = "true"
+	u.NotifyProps["push"] = USER_NOTIFY_MENTION
 	u.NotifyProps["desktop"] = USER_NOTIFY_ALL
 	u.NotifyProps["desktop_sound"] = "true"
 	u.NotifyProps["mention_keys"] = u.Username + ",@" + u.Username