Add :slack_application_url support

The idea is that it is more convenient to see slack notifications like this:

> Viktar Basharymau deployed twitterclone's master to [twitterclone-staging.example.com](http://twitterclone-staging.example.com)

than messages like this:

> Viktar Basharymau deployed twitterclone's master to staging

because the former is nice and clickable.