Issues 341 349 password field updates

Password field cleanup.

* Ensure "hide" text is centered
* Simplify show/hide button css selectors to be more compatible with IE8 (not completely, still use one `+` selector)
* Fix password underflow problem
* Set a background color on the show/hide button and position the button just right to cover up the password field.


see https://bugzilla.mozilla.org/show_bug.cgi?id=965237

fixes #341
fixes #349
issue #305