Move ChallengeReadme foreign key to Challenge object

[tschaffter]

• [x] https://github.com/Sage-Bionetworks/rocc-schemas/pull/201
• [x] https://github.com/Sage-Bionetworks/rocc-service/pull/194
• [x] https://github.com/Sage-Bionetworks/rocc-db-client/pull/29
• [x] https://github.com/Sage-Bionetworks/rocc-client-angular/pull/95

[tschaffter]

The next step is now to update the client.

[tschaffter]

Done