When sanitizing an activity changeeset, we iterate through all changes,

build a dictionary, and then iterate through that. This can cause the

second iteration to be in a different order from the first on some

versions of Python. While probably harmless in normal use, this does

cause sporadic issues with test runs.
This change switches the dictionary to be an OrderedDict, so that the

order is preserved.

Unit tests pass on all supported versions of Python.