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