Add an embeddable widget feature

[fallen]

This would allow organizations to integrate the petition right into their websites by the mean of a "widget".

That would technically be done via an <iframe src="https://petition.tld/petition//widget">

Need to think about the needs in term of customization (background colors?) for better integration in the website.
Maybe needs to be able to attach several widgets (with different config) to the same petition.
So that the same petition can be integrated on several websites with different color themes.

[fallen]

A start of implementation is in origin/widget branch.