CRM-14324 Editing a contribution credited to a PCP removes the link to the PCP

Steps to reproduce:

• Record a new contribution for a contact, who we will call the "Donor"
• In doing so, credit the contribution to a personal campaign page (PCP)
• Save, and go to view the contact record for the owner of the PCP, who we will call the "Supporter".
• Observe that in the supporter's contribution tab, an entry exists for this newly created contribution soft credit, and the personal campaign page previously specified is listed within this entry. Good.
• Go back to the donor, and edit the contribution.
• On the edit screen, don't change anything – just hit save.
• Refresh the supporter's contact page and go to the contributions tab.
• Expect to see exactly the same info as before – an entry for the contribution, referencing the PCP.
• Observe that the contribution remains, but in place of the link to the PCP, now is the text "(n/a)". Also, after editing the contribution, the total displayed on the PCP thermometer will go down.