TLS error page: add button to launch cert viewer

If the user experiences a TLS error, he needs some way to override it.
We can either give the user a button that allows him to click straight
through to the website, like Chromium does, or send him to a new dialog
designed to explain the implications of adding a security exception,
like Firefox does. Firefox's approach seems safest. (There are
additional possibilites, such as requiring the user to install the
certificate, which we can explore in the future, but for now let's
handle this in a more traditional manner.)

For now, use this button just to launch the existing certificate viewer.
This is useless though, since it contains too much confusing technical
information and does not actually allow you to add a security exception,
so the next step will be to either to use a custom version of the cert
viewer designed for this purpose, or to switch to a custom dialog
instead.