Adds Blind/anonymous review functionality

[chrisjrn]

This adds an option in ProposalSection to switch on Anonymous Review.

This allows blind review to be achieved in multiple ways:

• In symposion.reviews, returning proxied ProposalBase objects where .speaker() and .speakers() properties will return an object that is not the actual speaker, but will still display information in templates.
• By inspecting .anonymous_review() on a proposal, which can be used to completely hide fields referring to speakers whilst anonymous review is switched on.

The proxy object approach is somewhat of a blunt instrument, but it means that templates do not need to be correct in order to suppress the identities of speakers.

[candeira]

+1 good job Chris!