Add Grafana Oncall notification provider (#2783)

* Add Grafana Oncall notification provider

* Fix linter errors

* Remove useless variables

* Remove test message

Co-authored-by: Frank Elsinga <frank@elsinga.de>

* spelling consistency

* Update server/notification-providers/grafana-oncall.js

Co-authored-by: Frank Elsinga <frank@elsinga.de>

* Update server/notification-providers/grafana-oncall.js

Co-authored-by: Frank Elsinga <frank@elsinga.de>

* eslint requirements

Co-authored-by: Matthew Nickson <mnickson@sidingsmedia.com>

* Add Grafana Oncall translation

* Update src/components/notifications/GrafanaOncall.vue

Co-authored-by: Frank Elsinga <frank@elsinga.de>

* Check empty url

---------

Co-authored-by: Emmanuel Cohen <emmanuel.cohen@bso.co>
Co-authored-by: Frank Elsinga <frank@elsinga.de>
Co-authored-by: Matthew Nickson <mnickson@sidingsmedia.com>
Co-authored-by: Louis Lam <louislam@users.noreply.github.com>

src/lang/en.json

@@ -856,5 +856,6 @@
     "successDisabled": "Disabled Successfully.",
     "successEnabled": "Enabled Successfully.",
     "tagNotFound": "Tag not found.",
-    "foundChromiumVersion": "Found Chromium/Chrome. Version: {0}"
+    "foundChromiumVersion": "Found Chromium/Chrome. Version: {0}",
+    "GrafanaOncallUrl": "Grafana Oncall URL"
 }