rubric last modification time and associated fixes

Fixes #3329 (closed), perhaps should've done a separate MR but I did it in the middle of this work.

The main idea here is that rubrics know their last modification time.

• Fixes Issue #3296 (closed).

• display that in the Modify Rubric dialog (as well as the _edition (which I called "revision" in the UI).

• The diff rubrics collision dialog than shows you when it was last modified.

• I had to change a few tests...

  • increase test coverage if not sufficient coverage of new non-UI code

• split the conflict dialog to a new file

  • it hasn't changed much but has its own UI unit tests now (which is easier to do when you have separate class in its own file)