Send comparison of changes in PushBullet link

Instead of just having a handy link to the homepage, which doesn't really add anything, as if I want to view the site I know the URL, we could alternatively share a link to comparison of the changes on GitLab's comparison URL.

We need to be careful as the before_sha can be non-valid (i.e. when there's a pipeline that's been manually triggered, not as part of a push event) so in that case, we can still send the link to the site page.

Closes #43 (closed).