feature: Notify about failed updates, block detectable bad updates (#7188)

* Feature: Notify about failed updates, block detectable bad updates
Ability to post notifications when the update fails.
Detect and roll back updates that will cause broken installs. (Needs testing)
Add severity to notifications, critical (2) notifications will display a toast.

This will be used for removing in-tree dependencies and raising the minimum php version.

* Improve naming a bit add phpdoc to new_notification
In case multiple notifications are created, remove them all.

* Remove notifications when update is disabled.

* update travis to use db testing

* added missing index

sql-schema/205.sql

@@ -0,0 +1,3 @@
+ALTER TABLE `notifications` CHANGE `datetime` `datetime` timestamp NOT NULL DEFAULT '1970-01-02 00:00:00';
+ALTER TABLE `notifications` ADD `severity` INT DEFAULT 0 NULL COMMENT '0=ok,1=warning,2=critical' AFTER `body`;
+CREATE INDEX `notifications_severity_index` ON `notifications` (`severity`);