Fill in form action attribute in user_detail.html.

Using Chrome, the update form didn't submit with a blank action, but worked fine once I specified it.

Also, in researching whether a blank action attribute was OK, I found out that according to the HTML5 spec, the action attribute "must have a value that is a valid non-empty URL potentially surrounded by spaces."

http://www.w3.org/TR/html5/forms.html#attr-fs-action