add a :message attribute to Revision model.

resources/migrations/022_add_revision_message_field.yaml

resources/migrations/liquibase.json

src/metabase/api/card.clj

src/metabase/api/dashboard.clj

src/metabase/api/revision.clj

src/metabase/models/card.clj

src/metabase/models/revision.clj

test/metabase/api/revision_test.clj

test/metabase/models/revision_test.clj