Allow users to label Scans with multiple Points

[jpowie01]

Expected Behavior

Users should be able to label their Scans with points. Such points will be then assigned to a given tag.
Example task: "Using point please mark middle of the spine."

Tasks to do:

• [Frontend] New Selection tool that will allow to enter Points,
• [Backend] Extend existing structure and allow to add above Points to the Labels.

Points should be draggable! Everyone's not perfect :(

Proposal for JSON representation in the API (to be discussed):

{
  "x": 0.5,
  "y": 0.5,
  "slice_index": 0,
  "tag": "SPINE_CENTER",
  "tool": "SINGLE_POINT"
}

Proposal for UI design (to be discussed):