[FIX] ir_qweb: search assets as superuser

A security has been introduce in eb9113c04d66627fbe04b473b9010e5de973c6aa to restrict access to orphan attachments to employees only

Assets need to be build and accessed as superuser, thus
[FIX] email_template: don't escape the signature, it's a field `html`

Fields of html type are already escaped in database and the method
`append_content_to_html` defaults the argument `plaintext` to True,
then calls `plaintext2html` if this argument is True, resulting in
the html signature being escaped twice.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64