Add CachedCouchDocumentMixin to DomainInvitation

Made DomainInvitation cacheable as per the discussion in this pull request https://github.com/dimagi/commcare-hq/pull/3828.
@czue , I wasn't positive these were the proper values for `generation_key` and `doc_type`. Do they look ok? (at first I had `doc_types = ['DomainInvitation']`)