[FIX] Report: attachment and attachment_use: fix logic

Previously, attachment and attachment_use were only functionnal when used
simultaneously. The code did not consider the fact that the user may want
an historic of all the printed reports. This commit allow the use of
attachment without attachment_use.

It also tell explicitely qweb to generate the branding (used to separate
reports from a unique html when printing multiple reports). Before, it was
functional only when website was installed, because we forced 'editable' that
only has meaning in website.
[FIX] Report: attachment_use need attachment
[FIX] css: overflow-x: auto should be on modal-body