This commit allows clients to register client_gravatar=True, and
then we recognize that flag for message events. If the flag is
True, we will not calculate gravatar URLs and let the clients do
it themselves. (Clients can calculate gravatar URLs based on
emails with just a little bit of code.)
This fixes the original issue that #5598 was the root cause of; when
the user returns to a Zulip browser tab after they've been idle past
the timeout (10 min, per IDLE_EVENT_QUEUE_TIMEOUT_SECS), we now
correctly reload the page even if they're using Zulip in German or
another non-English language where we have a translation for the
relevant error message.