Add features and fix issue for Assignments/Tests

[achen318]

• Add time to "Due Dates" for both assignments or tests
• Not specifying a due date will set the due date to September 28th, 2020 [ISSUE]
• Add a dropdown menu for selecting Submission Methods, where commonly used platforms are included (e.g. Google Classroom, Jupiter, Turnitin), and the user can add custom platforms
• Add a 5-second period after checking off an assignment or test to undo completing it
• Ability to mark assignment priority to make it show on top of the list