CORE-5131 Fix for /save endpoint trashing files.

Updated the fileio.controllers/save function to rename the dest file before saving its new contents, since the Jargon API will attempt to delete the existing file before writing to it, which will cause the old version of the file to be put into the Trash.
Fixed validations in this save function to check the destination file rather than its parent folder.
Moved destination folder validation out of the fileio.actions/save function and into the store function which calls it, since it's also called by the fileio.controllers/save function.