ci: notify slack when master goes red

[mark-rushakoff]

Circle 2.0 supports slack notifications but doesn't seem to offer a way to only notify when one branch (i.e. master) goes red or is fixed.

It looks like there are some solutions out there to solve this, that involve running a whole separate heroku app or perhaps involving a specific outgoing webhook.

The specific motivation here was #712 and #682 were independently green, but after #712 was merged, #682 was merged cleanly but resulted in a compile error.

There's a separate discussion ongoing about whether we should require PRs to be rebased on master before merging, but even if that happens, we could still see red builds from flaky tests, and we should still receive notifications for that.

[gianarb]

I think we can use this plugin to have a badge in idpe readme that shows up the status of the master branch https://wiki.jenkins.io/display/JENKINS/Embeddable+Build+Status+Plugin

influxdb, telegraf has something similar already. So it should help

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