Decrypt base64 image

[chakeresa]

Changes proposed in this pull request:

• Changes createPhoto mutation to accept a base64 encoded string instead of an image file

Related to user story #57 & #59

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)
• [x] Merged in the latest master to my branch with git pull origin master & resolved merge conflicts
• [x] Ran rails db:migrate
• [x] Ran the test suite - all tests are passing (or maybe skipped)
• [ ] Checked affected endpoints in Postman / GraphiQL
• [x] Updated README for changes (new endpoints, new gems, etc)

Note: Testing in Postman is not trivial anymore, since it requires encoding the file first.