CDN: Adds support for serving assets over a CDN (#30691)

* CDN: Initial poc support for serving assets over a CDN

* Minor fix

* added build path and test

* fix lint error

* Added edition to cdn path

* Move master builds to a separate path

* Added error handling for the url parsing, changed setting name, and added docs

* Updated sample.ini

* Some property renames

* updated

* Minor update to html

* index template improvements

* Update docs/sources/administration/configuration.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Update docs/sources/administration/configuration.md

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

* Added ContentDeliveryPrefix to Licence service

* updated docs

* Updated test mock

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

docs/sources/administration/configuration.md

@@ -259,6 +259,15 @@ Path to the certificate key file (if `protocol` is set to `https` or `h2`).
 
 Path where the socket should be created when `protocol=socket`. Make sure that Grafana has appropriate permissions before you change this setting.
 
+### cdn_url
+
+> **Note**: Available in Grafana v7.4 and later versions.
+
+Specify a full HTTP URL address to the root of your Grafana CDN assets. Grafana will add edition and version paths.
+
+For example, given a cdn url like `https://cdn.myserver.com` grafana will try to load a javascript file from
+`http://cdn.myserver.com/grafana-oss/v7.4.0/public/build/app.<hash>.js`.
+
 <hr />
 
 ## [database]