[FIX] models: in onchange(), do not send a field value if it has not changed

The method onchange() executes onchange methods in cascade.  Suppose onchange()
is called and a field F=1 in the form.  If an onchange method set F=2, that
value is put in the result variable.  If another onchange method set it back to
F=1, the binding F=2 must be removed from the result variable.

Fixes #2309
[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)
[FIX] fields: in computed fields, handle AccessError and MissingError

When computing a field on a recordset, a subset of the records may be missing
or forbidden by access rules.  In that case, evaluate the compute method record
by record, and mark failed records as such in cache.
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64