* moved file moving logic to filemodels.File
* added unique file name/folder constraint
* s/LOGIC_PATH_EQ_ACTUAL/FOLDER_AFFECTS_URL/; it just sounds better
* removed unused imports
Added a 'last line of defence' check that ensures that no duplicate files can exist in the same fodler
Fix various issues regarding the enforcing of file unicity

* moved all clipboard unicity relatd validation in models.tools (there's still a certain amount of duplication, but I'll remove that in a future commit)
* removed move and copy actions; this would cause serious problems if recursivly moving/copying folders: if during moving/copying an exception is raised (ex due to connection problems with the storage backend, then we end up with an inconsitent folder hierarchy)
* removed _move_file_to_new_location and use the already existing _copy_file method
* only delete the previous version of the file AFTER the new version of the object has been commited to the db. This ensures that we'll won't end up having filer file objects pointing to non-existent files
* remove discarding feature and replace it with delete
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