V4S: Provide control over Account matching w/ NPSP

[davidhabib]

currently on the Sites page, if the user enters a Volunteer Organization that matches an account, and we are creating a new contact, V4S will use that account. This may not always be desireable, especially if the customer is using NPSP and one of the account models.

We should either provide a setting to disable this attempt at account matching, or just have V4S detect that NPSP is being used, and not set the account, so the NPSP default behavior will occur.

hub thread: https://powerofus.force.com/0D58000003Y4LVS

[davidhabib]

**lurch: add

[LurchTheButler]

Tracking W-019812