assignment errors if beneficiary org not set

Ran into an issue where we were getting errors when assigning volunteers to a need. I traced it to the createVolunteerActivity method, where the params were missing target_contact_id.

This is because no target_contact_id was stored in the volunteer_projects table, as no beneficiary org value had been saved.

The problem is that 1) that field is not required when enabling volunteer functionality; and 2) when the form loads, it fills in the default org for the system. but that value isn't actually saved – it's just a form default. that makes it very easy to "think" you have set the value, when in fact it hasn't been saved to the DB.

we either need to enforce a value (disallow assigning volunteers, etc. until a beneficiary is saved), or actually save the default org to the table and then let people change it as needed.