Resolve "Files as assignment descriptions"

What does this MR do (summary)?

Teachers can now upload a file for an assignment. The file can be downloaded from the assignment page. File can be deleted. Using .zip you multiple files can be uploaded.

Screenshots

Can upload file:

Can download file:

Can delete uploaded file (shown iff there is an uploaded file)

What changes have you made (detailed)?

• Can upload file (can upload multiple with .zip file) in add assignment and edit assignment.
• Can delete uploaded file in edit assignment (if it has been uploaded).
• Button to download assignment file in assignment page (visible to everyone in the assignment and only if there exists an uploaded file).
• Write tests

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.

Closes #21 (closed)