…sually different on initial load

- Add feature flag for locked ellipse labels
- Add labels to LockedEllipseType
- Update builder
- Update stories and tests

Issue: https://khanacademy.atlassian.net/browse/LEMS-2349

Test plan:
- Go to http://localhost:6006/?path=/story/perseuseditor-widgets-interactive-graph--mafs-with-locked-ellipse-labels-flag
- Confirm that the visible label is on the locked ellipse only
- Go to the point, line, and vector  stories and confirm that the
  locked ellipse does not have a visible label (make sure the
  flags are separating the behaviors correctly)

… this more complex than it needs to be

Add the ability to add/edit/delete labels for locked ellipses
within the interactive graph editor.

Issue: https://khanacademy.atlassian.net/browse/LEMS-2349

Test plan:
- Go to http://localhost:6006/?path=/story/perseuseditor-widgets-interactive-graph--mafs-with-locked-ellipse-labels-flag
- Go to locked figures
- Open the locked ellipse settings
- Confirm that the label settings are there
- Play around with the label settings and confirm that it moves
  and updates with the ellipse