Initial motivation for this rework was to add support for the responsive thumbnnails (#474). But it also is a re-implementation instead of an enahncement of the existing code because the moveMediaDirectory function had grown into a mess. It was very hard to debug possible renaming bugs, like https://board.s9y.org/viewtopic.php?f=3&t=21185. This approach uses several small functions instead that can be combined and re-used.
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