Fix "create annotation" for uint64 datasets

[normanrz]

Detailed Description

Fix "create annotation" for the datasets described below.

Context

The neuroglancer datasets (e.g. https://webknossos.org/datasets/Demo_Organization/h01_20210601/view#620659,317098,2662,0,91.819) have uint64 segmentations. When clicking "create annotation" there is an error.

Creating volume annotations with uint64 fallback layer is not supported by wK yet.

Creating skeleton-only annotations works, but is not what happens when clicking the "create annotation".

[fm3]

I see three options:
(1) For datasets with uint64 segmentations, make the default of the button skeleton-only
(2) For datasets with uint64 segmentations, make the default of the button skeleton + volume but without fallback layer
(3) Actually implement uint64 data #4852 (mostly front-end work, the back-end mostly has to remove some assertions)

[fm3]

closing this, as we decided to actually go for #4852