Make buildbot send the CS team the results from the nightly tests

Right now BB is putting test results in the -build channel. With the current process, the engineer has to remember to check it, then find the build of interest in a big list of builds that includes PR builds and nightly builds for every platform, then navigate BB's interface, etc etc etc. It's all pointless if nobody remembers to check in the first place, but if even they do it's a pita.

BB will be more useful if it tells us how the nightly tests went. To start, I'd like the C7 nightly builder for the develop branch to mail the CS mailing list its go.log file when it finishes. If there is no go.log file, meaning it didn't even make it to that point, we need to know that as well.

The msg summary should be "BB <distro> <branch> nightly results for <date>". If there's a go.log file, the msg body should be the go.log file. If there's no go.log file, the msg body should say "Test failed prior to running go.sh".

We'll start there. If this turns out to be useful, we can think about improvements. If not we can scrap it.