`sl pr submit` footer hard-codes reviewstack.dev — make the review link configurable

[netproteus]

Every stacked PR that sl pr submit creates gets a footer line:

Stack created with Sapling. Best reviewed with ReviewStack.

The reviewstack.dev URL and the tool name are hard-coded (eden/scm/sapling/ext/github/pull_request_body.py). That makes the link wrong or dead for:

• GitHub Enterprise users — reviewstack.dev can't see GHE repos at all (Reviewstack seems to always use "api.HOSTNAME" despite this not working for GitHub Enterprise #512), so every stacked PR links to a page that errors;
• teams running their own ReviewStack fork (Cannot build reviewstack #304) — which the ReviewStack source explicitly anticipates ("this URL can change over time (particularly if someone wants to run their own fork of ReviewStack)", eden/contrib/reviewstack/src/saplingStack.ts);
• teams using another stack-aware review tool entirely.

Related: #158 (footer URLs pointing at the wrong place for fork-based submissions).

Today the only knob is github.pull-request-include-reviewstack=false, which removes the line entirely — all or nothing.

Proposal: two configs, with unset behavior byte-for-byte identical to today:

[github]
pull-request-review-url-template=https://review.example.com/{owner}/{repo}/pull/{number}
pull-request-review-tool-name=MyReview

The URL template would support {owner}, {repo}, {number}, and {hostname} placeholders. The sentence shape (including the leading "Stack created with [Sapling]") deliberately stays fixed so that parse_stack_information() (used by sl pr pull), ReviewStack's client-side parser, and legacy pre-marker PR bodies all keep parsing unchanged.

I have a PR ready implementing this and will attach it shortly.