Refactor: Slack Notifications

[vdice]

We've currently swapped out all previous invocations of the Slack plugin with use of our own utility (located in this repo) after bumping into multiple issues with the former. One drawback w/ the utility (essentially a bash script using Slack's Incoming Webhooks API) and its use is it leads to verbose/ugly DSL.

Let's revisit and either get back to using the Slack Plugin (as it stands, even w/ latest 2.2 version, we still see errors thrown like this one) or bundle up said utility for use in a less verbose fashion.

[vdice]

I believe the latest version of the Slack Plugin now allows use of a Credentials ID secret for the slack token (whereas it didn't before, which was the first big motivator for using our own utility). It would be great to get back to use of the plugin as long as the error in the description is no longer hit.

[Cryptophobia]

This issue was moved to teamhephy/jenkins-jobs#3