[IMP] release: add coverage report to gh pages.

[SimonGenin]

Add a coverage report to the gh pages. Hopefully (can't really be tested without a release...), the path https://odoo.github.io/owl/coverage will contain the latest report, updated on each release.

If the script fails, it's not a big deal as it is the last thing of the release being executed.

[SimonGenin]

@ged-odoo I was wondering what was the coverage of the tests, so I added this little thing to try to add it to the github pages.

[ged-odoo]

@SimonGenin now I am curious. what's the result?

[SimonGenin]

It seemed pretty good actually