Add has_submittable_files to the exercises API endpoint

[jaakkonakaza]

Description

What?

Adds the field has_submittable_files to the exercises API, which is true for any exercise that has submittable files and else false.

Why?

Currently each exercise has to be checked separately to see if it is submittable, leading to a lot of unnecessary requests by the IntelliJ plugin. Having the field in the exercises endpoint speeds up loading and reduces requests.

How?

The form_spec in exercise_info is checked to see if any type is file. _generate_data in exercise/cache/basetypes.py does not have type info of exercises, so isinstance(lobj, ExerciseWithAttachment) does not work.

Testing

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

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Did you test the changes in

• [X] 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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