FileLockedOperation to prevent overlapping updates

[mbland]

For each site, a lock file will prevent any incoming updates from interfering with one another. Updates across multiple sites will still happen in parallel.

This will also enable us to move to an org-wide webhook publishing model, eliminating the need for individual repos to set up webhooks, without having multiple webhooks firing during the transition cause failures.

cc: @arowla (and mind peeking at 18F/team-api.18f.gov#108, too? ;-)

[arowla]

LGTM. Up to you whether to make the minor suggested changes.

[mbland]

Thanks, @arowla!