Add anchors

[cdaniel]

• [x] Add users to the map
• [x] Make users draggable
• [x] Make it possible to establish dependencies to other nodes
• [x] Make it possible to delete said dependencies
• [x] Ensure deleted nodes remove appropriate dependencies
• [x] Test the point above (map-schema # removeNode)
• [x] Support for diff
• [x] Make new changes downloadable
• [x] Make it possible to rename users
• [x] Inspect submap forming
• [x] Tests
• [x] Investigate why connection style is changed when diff is enabled.

[cdaniel]

Blocked by https://github.com/jsplumb/jsPlumb/issues/670

[cdaniel]

Freetext map titles will be tracked in a separate issue.