This changes `Corto.CreateIncident` to treat everything *but* the Phab
ticket creation as *best-effort*.  Additionally, creation of the
Google Doc, and templating & storage of the comment are now ran in the
background (I've seen these steps add significant latency to the
command).

This isn't 100% ideal; Any errors for these best-effort (and now
async) operations will be lost.

One option would be to return some sort of compound result set.  A
structure that could return all of the various successes and failures
to the caller.  Another would be to change the abstractions and move
logging into `Corto`.  My preference here would be for the latter, but
only after we transition Corto to a service (at which point an irc bot
is a mere client that doesn't need this sort of specificity).

For now, I think this is Good Enough™ (and it is
easy/straightforward).

Bug: T379858