[FIX] website_forum: respect karma when posting questions/answers/comments + more user-friendly errors

While posting new questions and answers the check
for karma limit was bypassed because it was using
super-user mode: use regular user instead.
Also improve the user feedback when karma level is
too low to perform some actions: post comment,
post question, post answer.
The usability in these cases still needs to be
improved.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64