Suport color for element types

Refs https://gitlab.com/teklia/requests/-/issues/857

Similar to the EntityType.color field, we want to add a new ElementType.color field, defaulting to the green color currently used on Arkindex #28b62c

The endpoints related to element types must be updated to support that field, especially UpdateElementType and PartialUpdateElementType.

Please also include the color field in ListCorpus, as it will be the main source of information when updating a corpus.