Feature undo schedule and resolve

[garylyp]

Changes made in this PR.

1. Implement undo-redo feature for ScheduleRegimeCommand, ScheduleCompleteCommand and ResolveCommand.
2. Add tests for the Events created for the above Undoable commands.
3. Observable lists are now sorted (except for the suggest list).

• Exercise List is ordered from most recent to least recent date. (decreasing date)
• Regime list is order by name lexicographically.
• Schedule list is ordered by nearest date to the furthest date into the future (increasing date)
• Can't think of a metric to sort suggest list yet so will leave it unsorted for now

[t-cheepeng]

In the future, if you wish to edit the PR description, just add a new comment so it is more visible. Thanks

[t-cheepeng]

Please resolve the merge conflict before it can be merged. thanks

[garylyp]

Sure. I was thinking it might be more visible if it's the first thing at the top of the page but actually anything works