Switched to use `display:none` in extra_tags_for_form method.

The use of `display:inline` with the content_tag call in the
extra_tags_for_form method potentially causes display issues with some
browsers, namely Internet Explorer. IE's behaviour of not collapsing
the line height on divs with ostensibly no content means that the
automatically added div containing the hidden authenticity_token, utf8
and _method form input tags may interfere with other visible form
elements in certain circumstances. The use of `display:none` rather
than `display:inline` fixes this problem.

Fixes #6403