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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[ihalaij1]

Nice!

I edited the code style a bit to make it cleaner.