speed up frontend bounding box rendering when user exclusively modifies thresholds

[ben-directai]

See discussion in #19

Old implementation set a low, fixed detection threshold on ALL detection classes. That way we only had to make one request and then we could suppress bounding boxes on the client side.

In this implementation we enable the user to modify the detection threshoids for each class. We can just set each class detection to 0 but we would be required to do NMS thresholding client side. worth considering