Automatically set `Sponsor.request_fulfilled`

https://osraav.atlassian.net/browse/OSRA-181

- "Request fulfilled" field is now disabled in the UI. Its value is set automatically by the application
- Sponsorship `after_save` (i.e. on creation & inactivation) calls sponsor's `update_request_fulfilled!`
- Sponsor `update_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`
  - updates attribute `request_fulfilled` with the return value of `request_is_fulfilled?`
- Sponsor `before_update` calls `set_request_fulfilled` - same as the `update` method above, but sets the `request_fulfilled` attribute on the in-memory object

OSRA-181 #Code-Review