Adds a "share" widget to the website forum

Task: https://www.odoo.com/web#id=8601&view_type=form&model=project.task
Pad: https://pad.odoo.com/p/r.ca4Qi9dcQZSnPQm9

Adds a social widget to website and social widget links to website_forum.

On website forum, a "Share" link is available for each question/answer.

When submitting a new question, a modal appears to promote sharing of the question.

See pad for details.