Add submissions zip download feature

[ihalaij1]

Description

What?

Add submissions zip download feature.

The submissions can be downloaded per exercise from the UI or API URL /api/v2/exercises/<exercise_id>/submissions/zip?best=yes

The zip archive contains a metadata file info.csv and submissions with a specific naming scheme:

• If the submission was made by a group, the file name starts with all of the group's student IDs joined by + signs.
  Otherwise, there is only one student ID.
• The student IDs are followed by an underscore and the file number, since some exercises accept multiple files.
• The file number is followed by an underscore and the number of the submission, since users can have multiple submissions to the same exercise.

Example file names in an exercise requiring two submitted files:

123123_file1_submission1
123123_file2_submission1
123123_file1_submission2
123123_file2_submission2
123456+987654_file1_submission1
123456+987654_file2_submission1

Why?

This was requested by multiple users.

Fixes #1374
Fixes #733

Testing

What type of test did you run?

• Accessibility test using the WAVE extension.
• Django unit tests.
• Selenium tests.
• Other test. (Add a description below)
• Manual testing.

Tested that the feature works with groups and multiple file submission exercises.

Did you test the changes in

• Chrome
• Firefox
• This pull request cannot be tested in the browser.

Think of what is affected by these changes and could become broken

Translation

• Did you modify or add new strings in the user interface? (Read about how to create translation)

Programming style

• Did you follow our style guides?
• Did you use Python type hinting in all functions that you added or edited? (type hints for function parameters and return values)

Have you updated the README or other relevant documentation?

• documents inside the doc directory.
• README.md.
• Aplus Manual.
• Other documentation (mention below which documentation).

Is it Done?

• Reviewer has finished the code review
• After the review, the developer has made changes accordingly
• Customer/Teacher has accepted the implementation of the feature

[murhum1]

Tried this out with varying numbers and file sizes of submissions, and they worked fine - I'll merge this.