Post the pull request report as a comment on it (#307)

[adrpo]

Follows #324, which landed the pull request testing of #307 with the summary only
written to a file and printed in the build log. This posts it.

Rebased on #327, so that one merges first; the commit to review here is the
second one.

--comment

./pr-report.py <N> --comment

posts the summary on pull request <N>, and replaces the comment an earlier
run of the same pull request left rather than adding to a pile - it recognises
its own by a marker in the body:

<!-- openmodelica-library-testing: pull request report -->

It posts as whoever the token belongs to: GITHUB_TOKEN or GH_TOKEN in the
environment, or, failing those, the account gh is
logged in as, which is what makes it usable by hand.

In Jenkins

The pull_request_comment parameter, taking the token from a github-token
credential. The credential is bound where it is used rather than for the stage,
so a job that does not ask for a comment runs whether one is configured or not.

Without a token nothing changes: the report is written and published as before,
the summary is in the build log, and the run says where the file is. With a token
that is refused, the run fails after the report has been written, so the mistake
is visible and a rerun - seconds, it retests nothing - publishes and comments.

Tested

Against a throwaway pull request in a fork,
adrpo/OpenModelicaLibraryTesting#1:

first run	posts the summary, as the account gh is logged in as
second run	updates that same comment, one comment on the pull request, not two
a token that is refused	says so with what GitHub answered, exits 1, report and index still written
no token at all	not an error: says where the summary file is and carries on

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