Make some fields on Speaker model optional.

[martey]

This makes it easier to add/edit Speakers in the admin.

• Make User relationship optional (since some Speakers will not have a User account associated with them if they are additional speakers to a proposal) by setting blank=True.
• Make annotation field optional (similar to the field on the Sponsor model).
• Make invite_email and invite_token fields optional by setting blank=True, since conventional Speakers attached to User accounts do not need these fields. This removes the uniqueness constraint from invite_token, but since null=True was set on that field (see https://docs.djangoproject.com/en/1.9/ref/models/fields/#django.db.models.Field.null), it wasn't really unique anyway.