Allow logged in users to use contribution / membership form with cid like with participant

One option we discussed for improving membership / pledge / contribution data entry was to extend the cid=0 concept to the contribution form.

I took a peak at the code & it was extremely easy to do this & all the flows I tested worked. However, unless someone else wants to do some testing I'll just leave this for tracking purposes. If someone does want to test I'm happy to do the code fix ups.

Flows I tested
1) logged in user does a pledge or contribution by credit card or pay later & it is applied to them
2) logged in user does a pledge or contribution by credit card with cid=0 in the url & a new contact is created (credit card & pay later, pledge & contributon)
3) logged in user does a pledge or contribution by credit card with cid=200 in the url where 200 is a contact they have permission to (credit card / pay later / contributon). It is applied to user 200.
4) logged out user puts either 0 or 200 in the url - payment is applied to them as if they hadn't meddled with the url

In all cases defaults filled correctly & contributions / pledges were assigned to the correct contact. No testing of mail.