Phase 3: a change request can be discussed

Phase 3: a change request can be discussed

Until now the only ways to say something about a request were editing its body or
approving it. A review thread is now a pushed file like everything else:

	requests/<ulid>/comments/<ulid>.md    author + created, then the body

It is the *same file shape* as a comment on an issue and is read and written by the
same code (`issues.ParseComment`, `issues.RenderComment`, with `requests.Comment` as
an alias), so the thread inherited the validation, the markdown boundary and the
ownership rule instead of reimplementing them. That last one is the part worth
having: `pre-receive` compares the author on the old blob to the pushing account, so
a review comment is its author's to edit or delete, an admin may repair a thread
(audited), and rewriting the author line in the same push does not launder it.

`vole request comment <id> -m` prints the comment's id and the page anchor
(`#c-<ulid>`), `edit-comment` and `rm-comment` are the author's own words, and
`request show` prints the thread so the ids are discoverable at all — an id nobody
can see is an id nobody can use. Adding a comment is free, like adding one to an
issue: a new file's author is a claim, and the boundary has never pretended
otherwise.

The request's own file stays outside the ownership rule, for the reason an issue's
does: state, head and base are shared, and updating them is the author's normal edit.

Verified: TestParseReadsTheDiscussion (a thread plus an approval in one tree,
ordered by id), TestCommentNeedsAnAuthorAndATime and
TestUnknownFileInARequestIsRefused (the subtree stays a closed format, with both
expected shapes named), TestCommentPaths (a request comment is not an issue comment
and not an approval); and end to end through the binary,
TestRequestDiscussionIsPushedAndOwned — the empty state prints the command, the CLI
adds a comment, the page renders it with its anchor and `request show` lists it, a
writer is refused rewriting, adopting and deleting somebody else's comment, may join
the thread and may edit his own — with the issue-comment empty state assertion added
to TestIssuePushedThenRendered earlier. scripts/ci.sh green (gofmt, vet, build,
markdown links, go test -race ./...).