sct-15: add-images modal (on new task creation) allows for filtering images by label

[naknomum]

• labels are presented as pulldown (both "custom" and ML labels)
• multi-select - does an OR match, so only one of all selected labels need be present on image to include it
• images contain multiple annotations, so only one annotation need to have a (single) label match
• filter-by-label used both in api to show image count and in background process adding images to task

:information_source: on hold for sct-10 to add custom labels.