Major rewrite of SecurePasswords

This is version 3.0beta1, the first of (many) betas before the final 3.0 release.
Additional features will be added in subsequent betas. Please see the extension page
on www.mediawiki.org for more information and a roadmap.

Change-Id: I5103c081e2725242be99f42451a0bec890c15f9b