Model creation namespace drop-down could use some UI improvement

[ghost]

The namespace drop-down in the model creation wizard is ugly, "static" and not easily searchable.

This is totally fine in most cases, because the expectation is that model creators will likely use only a few namespaces (e.g. their own private and maybe one public one).

However, if the user collaborates with a larger number of namespaces, the drop-down becomes a bit annoying (worse of course for all-powerful sysadmins who can do as they please with all namespaces).

We could re-use the combobox namespace search implemented in request forms, by also adding a REST endpoint to search namespace by partial name and specific roles associated to the searching user (in this case, either model_creator or namespace_admin).

This is mostly a low-prio cosmetic change, but it could improve UX in some edge cases.

[ghost]

This may also be applied (with a different role criterion) to the namespace drop-down in the import page.