Add "Split Segment" Tools

[hotzenklotz]

Generating ground truth from pre-segmented images requires voxel-wise split and merge operations. The anticipated workflow is to define a number of size-restricted bounding boxes that are automatically segmented and then corrected. Local merging is, in principle, already implemented via the fill tool, but needs some tweaks for this workflow.

This issue will create novel split tools, where users provide the split interface on several sections. These interfaces can be interpolated across multiple sections. WEBKNOSSOS will validate whether all relevant sections within a bounding bounding box have been annotated before applying the split in the box.
This feature will be added to the open-source version of WEBKNOSSOS.

Design Doc: https://www.notion.so/scalableminds/Design-Doc-Split-tool-152b51644c63809d85a2ca7b393229d4

Context

• Specific to long-running jobs (set jobsEnabled=true in application.conf)
• Specific to webknossos.org (set isDemoInstance=true in application.conf)