Handle better experiments lacking a design or make it non-null

[arteymix]

The experimental design can be missing and that is generally not handled very well in the codebase. When I first did some experimenting for loading CELLxGENE datasets, I had to populate an empty design or else the UI would be broken.

I think we should make it non-null able and give it an initial empty value in the model declaration. Keep in mind that the column itself cannot be non-null because that table also stores other kind of investigations that lack designs.

Another thing to look into is adding a unique constraint on the EE foreign key to prevent a design to be used by multiple experiments. This can be achieved with a one-to-one relationship. It also applies to the MV relation and GEEQ scores.

[arteymix]

There might be a case for experiments lacking a design such as hosting curated reference transcriptomes or cell atlases. If we decide to allow such datasets, we can revisit this and make the design optional. That would include hiding the design tab and returning 404 on the REST API design endpoints.