Fixed a regression in the user admin page introduced in a92e7f37.

a92e7f37 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`.