to a "constants" class

This patch was automatically generated as part of the following inspection:
https://scrutinizer-ci.com/g/owncloud/core/inspections/cdfecc4e-a37e-4233-8025-f0d7252a8720

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Issue #7111

add a optional parameter to skip check for expired files, this is necessary to find out to whom a file was shared after it was expired.

Use a DateTime object and have conversion handled by Doctrine instead of using
a date string with a fixed format.

pass the name of the item source from the browser to the server - no need to get the data via complicated db queries