Ensure dashboard is responsive

Before this change, the dashboard wasn't behaving responsively which was
a regression (I think) since 0.6.4. It looks like the change came from
the bootstrap upgrade to 2.3.2.

This change ensures responsive.css is loaded for the dashboard templates
and adjusts the padding on the body so content isn't flush to the
viewport edges.

The link to the dashboard is shown when in the narrowest viewport. It
was previously hidden.

A couple of minor FED gremlins are also squashed (hat-tip @andysellick).
[Backport] Allow minus character in ProductAttribute.code

I don't see a reason why it shouldn't be allowed. And as the Django
Admin is currently the only way to enter product attributes, supporting
it eases integration with the auto-slugifying that is enabled in
ProductAttributeAdmin.

Fixes #1308

(cherry picked from commit 4491582979a9515f555c56d714c67f2fdac81ea7)