Activity logs reflect all target contacts

http://dev.nysenate.gov/issues/6713

In the contact's changelog tab, an activity record will be displayed multiple times if the activity has multiple target contacts. this is because when we gather all the changes associated with the activity, each of those contact's attached to it are considered distinctly.

technically, that's a valid methodology. but is not very useful, and clutters the log tab considerably. we should limit the changes for activities to only changes specific to the contact (if the contact was added/removed from the activity, and if the activity data itself changed)

Brian's comment
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The issue is particularly painful when you have an activity that is attached to a couple hundred contacts or so.