In order to ensure we are notified of broken URLs, add testing of external
URLs as a step in the CI. It is added as `allow_failure` because it would
be inapproprate to stop merges because an unrelated link has broken.

Additionally, only test when CI is run on master branch to save repeatedly
testing the URLs each time a merge request or branch is updated.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>