Add CachedCouchDocumentMixin to DomainInvitation

Made DomainInvitation cacheable as per the discussion in this pull request https://github.com/dimagi/commcare-hq/pull/3828.
This looks right, though only because of this line: https://github.com/dimagi/commcare-hq/pull/3845/files#diff-16ce9f2eb3be0695a91eca75714bb3b7L2012

Were you able to verify locally that caching was working?