Fixed a regression in the user admin page introduced in a92e7f3.
a92e7f3 switched most of the internal stuff to format_html. Using format_html in the `render` method of `ReadOnlyPasswordHashWidget` caused it to generate `SafeString` instances. Later these safe strings where returned from `BoundField.__unicode__` which caused force_unicode to loose the "safe" information. This commit fixes that by ensuring that the render method returns `SafeUnicode` instead of `SafeString`.