messageList: Calculate gavatar url if avatar_url is null in message.
messageList: Calculate gavatar url if avatar_url is null in message.

After 663c99e0c026e04b6aa5e7310f1ab54a8ecc3537, mobile client
is registering for events stating that it is capable of calculating
gavatar url. So if user avatar is gavatar then server sends avatar
as null.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile