url_blocker.rb

- Extend Gitlab::UrlBlocker to allow relative urls (require_absolute
  setting).  The new `require_absolute` setting defaults to true,
  which is the existing behavior.

- Extend AddressableUrlValidator to accept `require_abosolute` and
  default to the existing behavior

- Add validation for ApplicationSetting#grafana_url to validate that
  the URL does not contain XSS but can be a valid relative or absolute
  url.

- In the case of existing stored URLs, validate the stored URL does
  not contain XSS. If the stored URL contains stored XSS or is an
  otherwise invalid URL, return the default database column value.

- Add tests for Gitlab::UrlBlocker to test require_absolute setting

- Add tests for AddressableUrlValidator

- Add tests for ApplicationSetting#grafana_url