django-storages 1.14.4 erroneous behavior for `exists()`

[mikewilli]

New version of django-storages (1.14.4) introduced behavior that breaks exists() when allow_overwrite is True. This behavior is going to be reverted, but there's no release with the revert yet. See this PR for more info: jschneier/django-storages#1422

For now we have reverted the library updates (https://github.com/mozilla/experimenter/pull/11569), and when django-storages gets an update we can try again.

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