Fix issues with last update bubbles and timestamp comparison.
We had some issues with timestamp comparisons that we never really

noticed, due to pages reloading on updates.
First of all, the Last Activity timestamp recorded in the template

matched the timestamp in the Last Update resource, due to how we

converted those two timestamps, but neither of those matched any

timestamp fields in any other API resources. This is because we

serialized the two former timestamps ourselves using isoformat(), but

the API uses DjangoJSONEncoder, which further normalized the timestamps.
Because of this, we couldn't compare timestamps from API calls to the

last activity timestamps. Now that Djblets's json_dumps uses

DjangoJSONEncoder for timestamps, we can use that instead of calling

the 'date' filter ourselves, and get some consistency across all

timestamps.
Now that that's done, we can mark the review request as updated when we

publish a reply, preventing an update bubble from appearing.

Set the Check Updates timestamp to be very low, and replied to a review.

Published the reply. I didn't see an update bubble, unlike before this

change. I verified through the API calls and template that the timestamps

were what I expected.