[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.