Mirror rust-lang/rust review labels, enable triagebot shortcuts and review-changes-since functionality (#6773)

* triagebot: use the `S-waiting-on-{author,review}` label sets

And configure triagebot review status label flipping.

Instead of
`pr-{not-reviewed,waiting-on-author,follow-up-review-pending}`, for two
reasons:

1. Consistency with `rust-lang/rust`. The `S-*` status labels are the
   ones that `rust-lang/rust` uses.
2. I don't think the `not-reviewed` vs `follow-up-review-pending`
   distinction makes a lot of difference in practice. However, we can
   still keep the `pr-follow-up-review-pending` label, since that is
   previously manually applied anyway.

* triagebot: allow users to relabel `S-*` and `C-*` labels

Through `@rustbot label`.

* triagebot: enable `review-changes-since` functionality

Basically, a web-rendered version of `range-diff` to make review after
force-pushes easier.

* triagebot: enable `shortcut` functionality

This allows PR reviewers and authors to easily use:

```
@rustbot ready   # S-{waiting-on-author,blocked} -> S-waiting-on-review
@rustbot author  # S-{waiting-on-review,blocked} -> S-waiting-on-author
@rustbot blocked # S-waiting-on-{author,review}  -> S-blocked
```

Author: jieyouxu

triagebot.toml

@@ -8,17 +8,52 @@
 [relabel]
 allow-unauthenticated = [
     "needs-triage",
+    "S-*",
+    "C-*",
 ]
 
-[autolabel."pr-not-reviewed"]
-new_pr = true
+# ------------------------------------------------------------------------------
+# General autolabels
+# ------------------------------------------------------------------------------
 
 [autolabel."needs-triage"]
 new_issue = true
 exclude_labels = [
     "C-tracking-issue",
 ]
 
+# ------------------------------------------------------------------------------
+# PR review
+# ------------------------------------------------------------------------------
+
+[autolabel."S-waiting-on-review"]
+new_pr = true
+
+[autolabel."S-waiting-on-author"]
+new_draft = true
+
+[review-submitted]
+# This label is added when a "request changes" review is submitted.
+reviewed_label = "S-waiting-on-author"
+# These labels are removed when a "request changes" review is submitted.
+review_labels = ["S-waiting-on-review"]
+
+[review-requested]
+# Those labels are removed when PR author requests a review from an assignee
+remove_labels = ["S-waiting-on-author"]
+# Those labels are added when PR author requests a review from an assignee
+add_labels = ["S-waiting-on-review"]
+
+# Adds at the end of a review body a link to view the changes that happened
+# since the review
+# Documentation at: https://forge.rust-lang.org/triagebot/review-changes-since.html
+[review-changes-since]
+
+# Allow reviewers and authors to easily flip between {`S-waiting-on-review`,
+# `S-waiting-on-author`, `S-blocked`}.
+# Documentation at: <https://forge.rust-lang.org/triagebot/shortcuts.html>
+[shortcut]
+
 # ------------------------------------------------------------------------------
 # PR assignments
 # ------------------------------------------------------------------------------