dp - Added canvas encryption to the backend and updated frontend accordingly.

[divyanipunj]

Closes #502

Deployed to: https://frontiers-qa3.dokku-00.cs.ucsb.edu/

This PR:

• Updates .env.SAMPLE to include APP_SECRET.
• Updates application.properties to include APP_SECRET
• Updates Course.java to include canvasTokenLastUpdated and canvasTokenLastFour.
• Updates CoursesController.java and CoursesControllerTests.java to reflect the encryption service added and new variables from Course.java.
• Adds TokenEncryptionService.java which adds a service with two methods that encrypt and decrypt canvas1 tokens.
• Updated CanvasService,java and CanvasServiceTests.java to decrypt the Canvas API token before use, since tokens are now stored encrypted in the database.
• Updates CanvasApiForm.jsx and CanvasApiForm.test.jsx to reflect the changes made in the backend.
• Adds db migration file 013-add-canvas-token-last-updated.json given the changes in Course.java.

As discussed, remove canvasTokenLastUpdated and canvasTokenLastFour from Course.java. Also remove db migration file, since it's not necessary anymore. Restore the frontend completely. Update CoursesController (to encrypt and decrypt the token dynamically) and tests accordingly.

[pconrad]

Closing, replaced by #676