Extract MessageDict.set_sender_avatar().
Extract MessageDict.set_sender_avatar().

We now populate the avatar url as part of the post
processing step of building message dictionaries,
so that the avatar url is no longer in cache.

This change makes the cache slimmer, because instead
of caching the avatar url (which often includes a long
hash), we just cache the smaller fields that are used
to compute the url.

Note that this commit still has the problem that we're
essentially computing the avatar url from cached fields
that can be invalid.  We will address that a few commits
later.

An immediate benefit of this change is that how we compute
avatar urls (or whether we compute them all) is now decoupled
from caching concerns.  We will address this later as
well.  (Some clients will be capable of computing their
own gravatar urls, for example.)

showell/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,showell/zulip,brockwhittaker/zulip,jackrzhang/zulip,kou/zulip,dhcrzf/zulip,jackrzhang/zulip,andersk/zulip,tommyip/zulip,jackrzhang/zulip,jackrzhang/zulip,andersk/zulip,mahim97/zulip,punchagan/zulip,brockwhittaker/zulip,andersk/zulip,dhcrzf/zulip,showell/zulip,brainwane/zulip,rishig/zulip,eeshangarg/zulip,synicalsyntax/zulip,eeshangarg/zulip,zulip/zulip,Galexrt/zulip,shubhamdhama/zulip,kou/zulip,rht/zulip,shubhamdhama/zulip,eeshangarg/zulip,kou/zulip,tommyip/zulip,shubhamdhama/zulip,rishig/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,timabbott/zulip,timabbott/zulip,rishig/zulip,dhcrzf/zulip,jackrzhang/zulip,rishig/zulip,tommyip/zulip,synicalsyntax/zulip,tommyip/zulip,punchagan/zulip,tommyip/zulip,rht/zulip,jackrzhang/zulip,punchagan/zulip,brockwhittaker/zulip,brockwhittaker/zulip,tommyip/zulip,mahim97/zulip,timabbott/zulip,Galexrt/zulip,kou/zulip,kou/zulip,rishig/zulip,dhcrzf/zulip,zulip/zulip,Galexrt/zulip,timabbott/zulip,mahim97/zulip,andersk/zulip,rishig/zulip,dhcrzf/zulip,Galexrt/zulip,Galexrt/zulip,Galexrt/zulip,hackerkid/zulip,mahim97/zulip,shubhamdhama/zulip,rishig/zulip,zulip/zulip,jackrzhang/zulip,eeshangarg/zulip,eeshangarg/zulip,timabbott/zulip,zulip/zulip,mahim97/zulip,showell/zulip,timabbott/zulip,brockwhittaker/zulip,showell/zulip,brainwane/zulip,punchagan/zulip,hackerkid/zulip,rht/zulip,dhcrzf/zulip,zulip/zulip,showell/zulip,timabbott/zulip,dhcrzf/zulip,brainwane/zulip,shubhamdhama/zulip,brainwane/zulip,rht/zulip,andersk/zulip,punchagan/zulip,rht/zulip,brainwane/zulip,synicalsyntax/zulip,Galexrt/zulip,hackerkid/zulip,punchagan/zulip,shubhamdhama/zulip,andersk/zulip,synicalsyntax/zulip,mahim97/zulip,synicalsyntax/zulip,tommyip/zulip,punchagan/zulip,eeshangarg/zulip,synicalsyntax/zulip,kou/zulip,synicalsyntax/zulip,shubhamdhama/zulip,andersk/zulip,brainwane/zulip,brainwane/zulip,kou/zulip,rht/zulip,brockwhittaker/zulip,hackerkid/zulip,zulip/zulip,showell/zulip