Remove client-side password crypto from JS tests

This removes client-side password crypto from the JavaScript tests.

In some JavaScript tests, it has been assumed that SHA-1 is used for the
password hash in user docs.  Those tests should, however, not rely on
implementation details of the user authentication hash function, as it
isn't the goal of those tests to check these.  Furthermore, this causes
problems when a password scheme is changed, or a new one is introduced.
Upgrade password hashes on authentication

We now upgrade user docs to the new PBKDF2 password scheme on successful
authentication if the password hash is still from the old days where we
only used plain SHA-1 for hashing salted passwords.

Closes COUCHDB-1780.