Add CourseSchedule class

[fhilgers]

This issue closes #22 (UI part is tracked in separate issue)

This adds implementation of CourseSchedule.
As it was explicitly named schedule and also the new ObjectHandler queries a table called schedules I have added this new table.
Each delivery is tracked in a separate table with a foreign key constraint on the schedule they are a part of.
I have also added constraints for common misconfigurations, e.g. a delivery date outside of the schedule or duplicate delivery dates.

As other required parts are missing (example: CourseProject, tracked in #53), I have not integrated the schedule into these parts.
Either this PR can stay open until all other requirements are met, or, preferably, this is merged and I'll do another PR integrating these changes afterwards.

Please let me know if I misinterpreted the requirements, I'll be happy to make changes.

How can this be tested?

cd server
npm run test

Screenshots

No UI changes were made.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have lowered the linter errors. Before: 17. After: 17

[joluj]

Thank you for your contribution!

As other required parts are missing (example: CourseProject, tracked in #53), I have not integrated the schedule into these parts.
Either this PR can stay open until all other requirements are met, or, preferably, this is merged and I'll do another PR integrating these changes afterwards.

If that's okay with you I'd wait for #62 (the former #53) to be merged first.

---

Nevertheless I'd rename deliveryDate to submissionDate or dueDate, as deliveryDate has a connotation related to logistics, not exercises.