Add update/delete endpoints

This will allow manual edits from the front-end, like adding metadata to existing records. (Necessary for https://github.com/maiera/gde-app/issues/13)

Some things to consider with the endpoints-proto-datastore library we are using.

-  There is currently no difference between update/patch methods, both of them do a `patch` internally, but,

- Repeated properties (like metadata) have to be always supplied in the post body, otherwise any existing values will be replaced

See: https://github.com/GoogleCloudPlatform/endpoints-proto-datastore/issues/65