fix: normalize enable_course_sorting_by_start_date to boolean type

[PKulkoRaccoonGang]

Description

Since we have the feature flag ENABLE_COURSE_SORTING_BY_START_DATE, which provides a boolean value, and we also receive the enable_course_sorting_by_start_date parameter (as a string) through the form-data in the /search/unstable/v0/course_list_search/ API, this PR introduces the normalize_bool function to ensure that the enable_course_sorting_by_start_date parameter is always a boolean before being used in conditional logic.

Before

The value of enable_course_sorting_by_start_date was always evaluated as True, because it was passed as a string ("true" / "false").

Now

Depending on the actual data type of enable_course_sorting_by_start_date, its value is normalized to a proper Python bool using the new normalize_bool function.

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[PKulkoRaccoonGang]

[inform]: Since distutils.util.strtobool is deprecated, a custom normalize_bool helper has been added.

[kyrylo-kh]

LGTM

[bradenmacdonald]

Makes sense. Too bad the API is using POST form values instead of JSON data.