Metadata validation does not respect "required" for any field other than "title"

[Arnei]

When creating a new event or series, certain metadata fields can be required, meaning they have to have a value before the user can proceed to the next step. Usually, only the title field for the common metadata catalog is required, but users can change that in the backend.

Unfortunately, our validation for metadata fields is hardcoded and as such only checks if there is a value for title. Should another field be marked as required, this will be correctly shown in the ui, but will not be respected by validation. For example, `description* may also be marked as required, but the user can proceed to the next step without having entered any description.

To properly solve this, we would need to make our static validation dynamic, based on the metadata catalogs schemas from the backend.