Consider removing <isindex>

Mozilla is thinking about removing support for HTML element. References: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/isindex https://groups.google.com/forum/#!topic/mozilla.dev.platform/DV3YBf7wI3M and https://bugzilla.mozilla.org/show_bug.cgi?id=1266495

The reason we might want to do this for TB 6.0 is that generates a form that has a label that contains text that comes from the browser's UI locale (thus leaking that information).

There is a risk that some sites are using this tag.