[IMP] website_forum: add a "share" widget & modal

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 (idem for answer).

See pad for details.

https://www.youtube.com/watch?v=spyXwo2sM28&feature=youtu.be

![screenshot from 2014-11-28 10 50 50](https://cloud.githubusercontent.com/assets/9530767/5226213/9c302740-76ec-11e4-8ad6-040a818a2583.png)