Import Activity History : Should not require activity_id

When I try to import activity history records without mapping an Activity ID column, I get the following error on all records:
"Required fields activity_id not found for history"

I can work around this by putting dummy activity ID values in my import file. However, for typical use cases, there won't be a "meaningful" activity_id reference for these - as they are coming from a legacy or other external app. Also, unless a module and callback value is supplied - we don't show a "Details" link and hence don't use the activity_id value. So I don't think we should require the activity_id - which also means changing the schema slightly so that activity_id allows NULL values.

( For 1.4 - we'll need to tell folks to put in "dummy" value unless they are actually using the "Details" link, etc. )