CRM-14197 'first step'

[JoeMurray]

No description provided.

[eileenmcnaughton]

So in terms of completeness - this allows us to remove the membership_payment & participant_payment tables. But it doesn't require this. It appears fairly safe & adds searchability. But it does not include a piece in the upgrade script to fill this field from prior data.

I didn't get to this on the sprint but think it could be merged if we had this part done
"
We will change the 'semantics' of the line_item.entity_table and line_item.entity_id for memberships so that it points to the civicrm_membership table and id for memberships rather than civicrm_contribution and id as it does now"

The membership_payment removal & participant_payment removal are desired tidy up but I don't think they have to be there pre-merge (ie. this is stable without those extra changes & until we make those changes there is no impact on the api)

[eileenmcnaughton]

I'm closing this in favour of #3240 which contains these commits rebased - please take discussion there