Bugfix: Inconsistencies in share expiry dates

* Share expiry dates now always refer to the end of the given day. This change allows users to select the current day as expiry date.
* Displayed expiry dates have been aligned to ensure their consistency.
* Existing expiry dates for public links can now be removed again.
* We now use the Luxon `DateTime` object more consistently across the code base (replacing JavaScript's `new Date()).

https://github.com/owncloud/web/pull/6084
