New features in plan UI for enhanced usability

[j-atkins]

Following on from #277, and related to #258, this PR adds more features to the plan UI, designed to make the user experience easier and avoid cumbersome elements.

Additions include:

• Buttons for adding/subtracting 1 day, 1 hour or 30 minutes to waypoint times.
• Button for deleting a specific waypoint (with an "are you sure?" screen), may be useful in light of the new 'problems' module (Adding 'problems' module #269).
• "Reset changes (all waypoints)" button, which can be used if the user wants to revert back to their original schedule.

---

So the schedule editor portion now looks something like: