[Add]: structure flow to create the assignment new [new pr]

[Zzz212zzZ]

General Info

• Pivotal Tracker Story
• Breaking?
• This deprecated pr messed up with many unrelated commits. So I created a new branch and made this pr with clean commits, plus amended the code according to the comments in deprecated pr.

Changes

Explain your changes here (in such a way that you would understand why you made them a year from now).

1. Adjust the Assignment Table. Now it should rely on Course_to_lms Table instead of lms Table.
   
2. Implement the create method within the assignment controller

Testing

• Validate if the parameters are not int.
• The assignment can be created only when both the course and lms exist before. It uses a foreign key of course_to_lms_id to ensure this.
• We also do not want to create a duplicated test, i.e., there shouldn't exist such a record that has the same course_to_lms_id and external_assignment_id that we want to create.

Documentation

Updated E-R diagram

Checklist

• Name of branch corresponds to story

[Connor-Bernard]

LGTM