Initial draft for m2 version history

--no-rebase

Initial draft to add m2 to the OMERO version history, could do with @qidane @jburel and @joshmoore looking over it and suggesting additions/modifications - I skimmed PRs but wasn't entirely sure what was most relevant for developers.
from the OMERO side the biggest infrastructure change is that the units have "real enums". This is the new doc page that I was mentioning to you so we could link there (if we do that from history.txt)