Makes it easier to customise proposals

[chrisjrn]

In at least one conference I've worked on, we needed to fork Symposion to remove the description field on ProposalBase, as we didn't use it.

This PR makes description, abstract, and additional_notes blankable, and adds a convenience mixin for ModelForm that lets you make these fields required again.

This preserves original functionality, and also makes it possible to customise/disable fields from BaseProposal without needing to fork Symposion.

[jhrr]

This all looks reasonable to me. I'll consult with @jtauber and hopefully be able to get this merged into master on Monday.

Thanks for the effort!