Add /user endpoints

[benjaminkostiuk]

Overview

Implement /user endpoints for the course-management microservice defined in https://github.com/puffproject/docs/blob/master/API_ARCHITECTURE.md.

The task breaks down into the following steps:

PR#1 should be:

1. Create an AssignmentEnrollment entity based on the ASSIGNMENT_ENROLLMENT table. This is the table that keeps track of if a user is tracking an assignment, i.e. they are working on it. This includes a column where they can pin the assignment for easy access. Should join on the assignment id (See Assignment.java to see example of join column)
2. Annotate the AssignmentEnrollment entity with swagger annotations (@model etc.) + ignore the id and user id fields.
3. Create an AssignmentEnrollmentRepository for the JPA database connection.
4. Create an AssignmentEnrollmentPage model (extend BasePage) to define the data model we want to return to the user. Add necessary swagger annotations.
5. Add test data in the bootstrap_data.sql file to automatically populate the database when the service is loaded.

PR#2 should be:
5. Create a user service. You'll need methods for

• Getting the user's assignment enrollments (Needs to have a pageable and non-pageable versions)
• Enrolling a user in an assignment with the option to pin the assignment for them. Calling this endpoint multiple times should not ever create multiple records. There should only ever be one enrollment for a user in an assignment.
• Unenrolling a user from an assignment
• Check if a user has already voted on something (Might have to create a new function in a the vote service)

6. Create a Rest interface for user endpoints in restApi package with all the right annotations. Don't forget to include the Principal argument to get the user's token (it represents their identity)
7. Implement the rest interface in a user controller. The controller will call the user service endpoints to implement:

• GET /user/assignments: pageable endpoint that returns AssignmentEnrollmentPage objects. (Double check here that we can sort to the have the pinned ones at the top, I'm not sure if it's possible)
• POST /user/assignment/{assignmentId}/enroll?pin={true|false}
• DELETE /user/assignment/{assignmentId}/unenroll
• GET /user/engagement/{sourceType}/{sourceItemId}/vote: Check if a user has voted on something (Might need to create a new model to return)
• Manually test each of the endpoints + they should appear in the swagger under the User tag.

PR#3 should be:
8. Automated testing for the user service class.
9. Automated mock testing for the user endpoints.

[benjaminkostiuk]

@youcefs21 Just take this one step at a time. You can reference the exisiting code or ask me for anything you don't understand. Most of the code should be fairly similar to what's already there but no guarantees.