create zulip thread for every PR adding a new proposed project goal

[nikomatsakis]

One thing that was annoying in running the goals program was that when people open PRs proposing a new goal, there was no Zulip thread automatically created. I think it'd be nice to create a Zulip thread in #project-goals -- or maybe in a new #project-goals/proposed stream.

To implement this requires modifying triagebot. I've opened a corresponding issue there and I'll link it here.

[nikomatsakis]

Mentoring instructions

See rust-lang/triagebot#1880