[FIX] website: editor: ignore spurious changes for editable content

Changes to contentEditable or attributeEditable attributes
should not cause the corresponding section to be marked
as dirty (oe_dirty). This would otherwise cause an extra
editor save() for those, wrongly marking untouched
templates as `noupdate`, and possibly triggering access
right errors.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64