R1:69445222f7d9

Track content sources (email, web, conduit, mobile) for replies

Summary:
When an object is updated, record the content source for the update. This mostly
isn't terribly useful but one concrete thing I want to do with it is let admins
audit via-email replies more easily since there are a bunch of options which let
you do hyjinx if you intentionally configure them insecurely. I think having a
little more auditability around this feature is generally good. At some point
I'm going to turn this into a link…