open Zulip streams for new project goal proposals

[nikomatsakis]

Per rust-lang/rust-project-goals#216, it'd be great to have a new Zulip stream for each PR that adds a new project goal proposal.

Today, the bot detects the creation of new tracking issues and creates a corresponding Zulip thread:

triagebot/src/handlers/project_goals.rs

Lines 222 to 228 in 38b904f

	// When a new issue is opened that is tagged as a tracking issue,
	// automatically create a Zulip topic for it and post a comment to the issue.
	Event::Issue(IssuesEvent {
	action: IssuesAction::Opened,
	issue,
	..
	}) => {

We would presumably want something very similar. I'm imagining that if a new PR is created that adds a new file matching the regex src/\d\d\d\dh\d then we open a Zulip stream like proposal: XXX rust-project-goals#123 with a link to the PR.