Automatically set `Sponsor.request_fulfilled`

https://osraav.atlassian.net/browse/OSRA-181

- each time a sponsorship is saved (i.e. on creation & inactivation), it calls sponsor's `set_request_fulfilled`
- `set_request_fulfilled` 
  - calls `request_is_fulfilled?`, which checks whether the number of active sponsorships is equal to or greater than the number of requested sponsorships and returns `true` or `false`
  - sets attribute `request_fulfilled` to the return value of `request_is_fulfilled?`