Undo/redo

https://github.com/OpeningDesign/draw/issues/11

• Store previous changes for each drawing separately - in a history.json file for files and a history table for database storage(?).
• Have config to limit the number of changes stored
• Assign version id to each element that changes with each change to detect change conflicts
• Undo menu shows all changes, including those done by other users
• Undo button just undoes your changes
• ability to drop changes if no changes built on top of it (could either revert the change or delete the change)