Add "Download all points" endpoint and button

[jensjvh]

Description

What?

Add a "Download all points" button for exporting all submission points to a file. Also increase the submission modal size in the base.html template.

Why?

Enable teachers to export submissions that are still pending or have an error.

How?

Add a new endpoint CoursePendingSubmissionDataViewSet which uses the filter_best_submissions() function for getting all submissions regardless of their status.

Fixes #1508

Testing

Remember to add or update unit tests for new features and changes.

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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.

[ADD A DESCRIPTION ABOUT WHAT YOU TESTED MANUALLY]

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

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[ihalaij1]

Excellent!

I tested this and it works great.