[FIX] web: read_progress_bar: convert dates to strings
[FIX] web: read_progress_bar: convert dates to strings

Since rev. 960360af, Date (resp. Datetime) fields return
datetime.date (resp. datetime.datetime) objects. This caused an
issue in the read_progress_bar method which returns a dict, whose
keys are values of the group_by. Indeed, when grouped on a Date or
Datetime field, the key was no longer a string but a datetime.date
or datetime.datetime object, and it crashed when that dict was
turned into a JSON string.

This method isn't tested at all for now, but tests will come
shortly.

Task 1878254.

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