[IMP] doc: various improvements to website tutorial

closes #3476
closes #3477
closes #3479
[FIX] forum: closing spam/offensive questions should give author karma penalty

This is a partial patch for issue #3460, pending more
improvements and refinements in master.

Currently the karma penalty is hardcoded to 5*downvote penalty,
which may or may not be sufficient to prevent posting, depending
on the other karma levels.
[FIX] longpolling/gevent: log errors occurring in main loop
[FIX] longpolling/gevent: automatically restart worker when killed

Whenever the longpolling/gevent worker dies, it
should be automatically restarted by the prefork
server, just like it happens for HTTP workers.
[FIX] doc: typo missed when merging contribution
[IMP] safe_eval allow math
[IMP] sale_order_dates: quotation views (backport of b5e9e11)

Add the same fields on the quotation tree view than on the sale order tree view.
[FIX] website: correct padding of footer