Restrict date string query parameters to ISO 8601 format

Some endpoints of the web application take date strings as query parameters.

Currently, every kind of date strings parsable by dateutil.parser.parse are allowed to be provided.

This is not really consistent with other swh modules that only manipulates ISO 8601 date strings.

The web application should only allow to pass ISO 8601 date strings as query parameters and the dateutil dependency should be removed.

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Migrated from T2375 (view on Phabricator)