improve UI/UX for removing runs from the schedule without deleting them [#186669498]

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Contributing to the Donation Tracker

• I've added tests or modified existing tests for the change.
• I've humanly end-to-end tested the change by running an instance of the tracker.

Issue from Pivotal Tracker

https://www.pivotaltracker.com/story/show/186669498

Description of the Change

Both some frontend and backend changes here, firstly the frontend displays a different icon rather than 'X' for removing a run from the schedule (it implies deletion when that's not the case), and adjusted the current move functionality to accept null for the destination, which means to remove the run from the schedule, rather than explicitly setting the order to null via the edit api. It makes the move logic a little more centralized. I still plan on making it a v2 run verb, but the edit api for runs on v2 isn't complete yet, so I'll probably handle it as part of that.

The primary purpose of this change means that removing a run from the schedule via the command api correctly fills in the hole for starttime for any runs that were after it, but it also help with general cleanup.

Verification Process

Dragging runs around with the schedule editor works as expected, especially when removing runs from the schedule.