Prevent removal of all users from dataset permissions

[effigies]

Is your feature request related to a problem? Please describe.

It is possible for the sole maintainer of a dataset to remove themselves from the dataset.

Describe the solution you'd like

Attempting to remove all users (or the last user; if there's no such thing as batching) with permissions to a dataset should cause an error.

This has the corollary that users should not be able to be deleted if they are the only user with permissions on a dataset. They need to assign a successor. In the worst case, we could have them hand over control to a site admin and make it clear that we will assume editorial control for the dataset and may make changes or assign a new maintainer for the dataset.

Describe alternatives you've considered None.

Additional context Had a user delete themselves and ask to be re-added.