Make zarr_dir optional when creating dataset

[tcompa]

• Starting from fractal-server 2.8.0, zarr_dir becomes optional in the dataset POST.
• If user has no project_dir in their settings, zarr_dir becomes required (backend-side).
• In the dataset-creation modal, let's add a description like "The main folder for OME-Zarrs of this dataset. If not set, a default subfolder of {user_settings.project_dir} will be used."
• Upon successful dataset creation, display a temporary message showing the value of response.zarr_dir.

[jluethi]

This sounds great.

Once we validate that those defaults are working well in practice, I'd like to make it a stronger default for the regular user flow: e.g. a user that has {user_settings.project_dir} set sees Dataset name & Advanced options. They'd have to expand Advanced options to set the optional zarr_dir to something else.