add keybind to set brush color to unused value about deepcell-label HOT 2 CLOSED

Added in npz mode. Uses map to store the maximum label encountered in each feature. Most cases where a new label needs to be generated are handled in caliban.py, which also keeps track of similar information. In this case it seemed worth it to store those values (somewhat redundantly) in a javascript object to prevent server calls when changing the brush value, and to avoid passing extra information in the payload, when payload.tracks contains sufficient info to calculate the max labels in each feature.

Note: Math.max(... list) is used to get the maximum value in the list, but documentation notes that this may fail if too many elements are in the list. Unclear whether our data will ever reach "too many elements" for this to work, but if that is the case we may need to get the max from array with a slightly different approach.

from deepcell-label.

Added for npz files in #63 and for trk files in #72

from deepcell-label.