update our revert-to-revision protocol function to require passing in a user-id which is necessary for most entities and add revision/revert endpoints onto our dashboard api with unit tests.

update the functions for reverting a dashboard so that it handles restoring DashboardCardSeries now.

update implementations for serialize-dashboard and diff-dashboard to account for the addition of :series on DashboardCards.

when no field-values exist for a field, clear out any previous field-values so we don't leave stale data lying around.

apply a check for `valid-metadata?` when updating the attributes of a Field so that we can prevent invalid metadata configurations (#824)

adding an api endpoint for GET /api/dataset/card/:id which runs the given Card's query, then returns the result along with the card details.  also did some updating on the unit tests.

update api endpoints for adding/updating DashboardCards to allow for specifying the `:series` of additional cards to apply.

add the DashboardCardSeries model which allows attaching additional cards to a DashboardCard.  this includes the new database migration, new model code, new persistence functions on DashboardCard, and a bunch of unit tests.

update api endpoint GET /api/dashboard/:id to remove the superfluous wrapper with {:dashboard {real thing we want}}

move api endpoint POST /api/dashboard/:id/reposition -> PUT /api/dashboard/:id/cards so it's more consistent with the other dashcard api endpoints.

tidying up dashboard api unit tests a bit before we start.  also added a unit test for the reposition endpoint, which was missing.