[FIX] web: modals: overflow-x on modal-body to prevent large content to be displayed outside
[FIX] web: about odoo: correct bootstrap structure inside the modal

Bootstrap's container class has a fixed width, using it in a modal
with the previous commit allowing an horizontal scrollbar makes the
layout goes crazy. Use container-fluid instead and cleaned some css.
[FIX] website_forum: header template: double display of questions

The <t t-raw='0'/> is already defined lower in the view (line 101)