Only show member's image if it exists

Related to https://github.com/openaustralia/publicwhip/issues/406

Introduces:
- [x] a service class for checking if a resource exists
- [x] modifications to `Person` to include methods that check if the resource associated to the large and small image URLs exist
- [x] usage of these methods in the `member#show` view
- [x] webmock for testing