MB 21: Disable the password manager

This commit disables the about:login page and removes the "Login and
Password" section of about:preferences.

We do not do anything to the real password manager of Firefox, that is
in toolkit: it contains C++ parts that make it difficult to actually
prevent it from being built..

Finally, we modify the the function that opens about:login to report an
error in the console so that we can quickly get a backtrace to the code
that tries to use it.