Get Mochitests running for each a-s github commit/PR/merge

[dmose]

This could be as simple as doing a push-to-try for each commit that shows up on a relevant PR, or else having a named branch off of mozilla-central and pushing to that. I'll be talking to a-team folks to see what the preferred solution is.

[Mardak]

Some notes of manual steps from running the script on my own account:

• first run build artifact build confirming yes
• add mk_add_options AUTOCLOBBER=1 in mozconfig
• add pine as a hg remote in hgrc
• add ui email/username in hgrc
• add user/StrictHostKeyChecking no for hg.mozilla.org in .ssh/config
• edit +const OLDEST_PR_DATE = "2017-02-20";

Other than actually setting up a cronjob, the changes at https://github.com/dmose/activity-stream/tree/export-merges seem to be working as per #2195 (comment)

[dmose]

Resolved by #2198.