Add general comments ability to Review Bot

Review Request #10335 — Created Nov. 27, 2018 and submitted — Latest diff uploaded




Continuation of work from here:

Similar to how a tool can post diff comments on a file, they
can now also post general comments on the review. This also
paves the way for tools to post file and screenshot comments
(but does not add this functionality).

Was able to post an automated review with/without general
comments, opening issues on some comments and not on others.
Also posted an automated review with 35 general comments and
1 diff comment -> only 30 got posted, all of which were the
general comments.
Likewise posted an automated review with 15 general comments
and 17 diff comments -> only 30 (total) got posted, 15 general
and 15 diff.
Additionally, tried using an updated Review Board server with
a worker without the changes -> No errors, everything works
as expected.