refactor: Extract new_recipient_ids local.
refactor: Extract new_recipient_ids local.

We rename needs_new_sub (which sounds like
a boolean!) to new_recipient_ids, and we
calculate it explicitly within the loop, so
that we don't need to worry as much about
subsequent passes through the loop mutating it.

This allows us to also remove recipient_ids,
which in turn lets us remove recipients_map,
albeit with a small tweak for stream_map.

I also introduce the my_subs local, which
I use to more directly populate used_colors,
as well as using it as the loop var.

showell/zulip,zulip/zulip,rht/zulip,showell/zulip,zulip/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,zulip/zulip,showell/zulip,punchagan/zulip,hackerkid/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,kou/zulip,zulip/zulip,zulip/zulip,punchagan/zulip,zulip/zulip,rht/zulip,kou/zulip,punchagan/zulip,andersk/zulip,rht/zulip,showell/zulip,showell/zulip,rht/zulip,hackerkid/zulip,andersk/zulip,zulip/zulip,eeshangarg/zulip,kou/zulip,showell/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,andersk/zulip,kou/zulip,hackerkid/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,punchagan/zulip,kou/zulip,rht/zulip,showell/zulip,andersk/zulip,rht/zulip,eeshangarg/zulip