JIRA ticket: LIMS-1807

Summary:

To make it easier to mark lots of wells, the 'Mark Dispensing' button should stay active until explicitly cancelled.

Changes:

• Remove trigger and listener when a dispensing point has been marked
• Change 'Cancel' button to 'Finish' to match the Mark Point and Mark Region button equivalents

To test:

• Go to /admin/imaging, then click on a container with some subsamples
• Go to a well with subsamples, there will probably be a dispensing point marked already (or maybe several)
• Click 'Mark Dispensing', check the button changes to 'Finish' (and there is also a 'Delete' button)
• Click somewhere on the image. Check the (most recent) dispensing point moves to where you clicked.
• Check the buttons still say 'Finish' and 'Delete'. Move the dispensing point again and check it still moves.
• Go to a different well with subsamples, check the mark dispensing is still active, so you can quickly go from well to well and mark the dispensing point in each.
• Check the 'Finish' button works as expected, and that clicking on the image no longer marks the dispensing point.