Tear out OpenId Provider code, and implement simple Developer/Test OpenId Provider

The OpenId Provider we currently use for development and testsuite environments is needlessly complex, from when Launchpad was itself an OpenId Provider. We should tear out all of this code, implementing a minimal OpenId Provider. The login screen can just be a single form containing:

 * a drop down list of the email addresses in the EmailAddress table.
 * a text box to use an email address not in the EmailAddress table.
 * a text box to override the OpenId Identity to use.
 * a login button.
 * a fail to login button.

Doing this, we remove a load of code. It allows us to drop the AccountPassword and numerous legacy OpenId tables. It allows us to refactor the Account/EmailAddress/Person/OpenIdIdentifier parts of the data model.

This is step 2 of https://dev.launchpad.net/LEP/OpenIdRoadmap, but also tech debt it its own right.