Undo/Redo functionality for planning/timeline actions

[dandelany]

Describe the feature request

We'd like to have Undo/Redo in the UI for actions performed on the Plan Timeline, eg. adding/changing/removing activities from a plan.

@ivydeliz has implemented this on a different app and has a plan for how to proceed incrementally on this. We'll plan to:

• Discuss with @duranb and @AaronPlave
• Build a framework in the UI for handling undoable/redoable actions
• Incrementally refactor existing timeline actions to support undo/redo - this doesn't have to be done all at once and could be feature flagged until ready for release.