Previously, each individual caller of load_messages that passed
num_before > 0 would do its own manual management of fetch_status;
now, we just do it inside load_messages.
This commit prepares the frontend code to be consumed by webpack.
It is a hack: In theory, modules should be declaring and importing the
modules they depend on and the globals they expose directly.
However, that requires significant per-module work, which we don't
really want to block moving our toolchain to webpack on.
So we expose the modules by setting window.varName = varName; as
needed in the js files.
This reverts commit bcdd12773e.
We need to do some improvements in handling FetchStatus for initial
narrows before this will be safe to deploy in production.
If individual messages arrive before we get the latest
messages from the server, they can create gaps in rendering,
and would often be offscreen anyway, so we just ignore them.
