Basic AWS S3 setup, add Photo model

[chakeresa]

Changes proposed in this pull request:

• Add the aws-sdk-s3 gem (resolves #23)
• Fixes factory bot for the seed file
• Add Photo model / table / factory / test (resolves #27). There is a polymorphic association: a Photo belongs_to either a User or a Dog.
• Add PhotoType
• Add photos field to DogType & UserType

The following checks have been completed:

• [x] Tested my new feature(s) as well as any feasible edge cases (if possible)
• [x] Checked coverage/index.html - did not add any new code that's not covered by testing (if possible)
• [ ] Merged in the latest master to my branch with git pull origin master & resolved merge conflicts
• [ ] Ran rails db:migrate
• [ ] Ran the test suite - all tests are passing (or maybe skipped)
• [x] Checked affected endpoints in Postman / GraphiQL
• [ ] Updated README for changes (new endpoints, new gems, etc)

Notes:

• I added the new ENV vars to Heroku, but you'll need to add them to your config/application.yml file locally.