Support mail address choice for OpenID SReg.
Assert for valid value choice in OpenID/SReg.

This is to avoid people trying to hack in a different e-mail than ones
listed in LDAP.
Merge remote-tracking branch 'origin/master' into openid-week5
Fix doing OpenID auth with no SReg.
Fix error when requesting unsupported SReg attrs.
Pass 'next' properly within the login form.
User proper <label/> elements for SReg items.
Print privacy policy URL for SReg when supplied.
Do not use 'SRegRequest' in views.

Use more abstract interface instead. This will allow us to implement
Attribute Exchange on top of it.
Support simple AX fetch request responses.
SReg/AX: ask only for available person data.

That is, don't ask for things we don't have a value for anyway.
Merge remote-tracking branch 'tampakrap/birthday' into openid-week5
SReg/AX: ignore empty/null values as well.
Fix handling AX-less requests.
Support storing user data preferences in the db.
OpenID: support automatic authentication for the same site.
Merge pull request #30 from mgorny/openid-week5

OpenID / week 5