This is a pure refactoring that mostly just moves code from
subs.js to the new stream_color.js and updates module references
accordingly. In order to prevent introducing some exports,
update_stream_color was given an additional "sub" parameter
and update_stream_sidebar_swatch_color was given an "id"
parameter.
Killed off unused initial_color_fetch var.
(imported from commit b7644ce67f50d31fb46f564d758d661eea776aa6)
This includes a hack to preserve humbug/backends.py as a symlink, so
that we don't need to regenerate all our old sessions.
(imported from commit b7918988b31c71ec01bbdc270db7017d4069221d)