Fix html validation errors (Issue 48)

http://html5.validator.nu/?doc=https%3A%2F%2Fharthur.github.io%2Fbugzilla-todos%2F

    Error: Stray end tag img.
    From line 175, column 68; to line 175, column 73
    Bugzilla"></img>↩

    Error: Element form not allowed as child of element span in this context. (Suppressing further errors from this subtree.)
    From line 179, column 11; to line 179, column 57
    <form id="login-form" onsubmit="submitLogin()">↩

    Error: Stray end tag span.
    From line 184, column 7; to line 184, column 13
    an>↩ </span>↩ <

    Error: Bad value true for attribute hidden on element iframe.
    From line 210, column 3; to line 210, column 64
    </div>↩↩ <iframe id="submit-iframe" name="submit-iframe" hidden="true">↩ </i