Athlete Subdirs - improvements

.. added /BAK subdir where all actvities .BAK files are stored
.. do not delete any original files in /imports /downloads when deleting activity
.. copy deleted activity .JSON as .BAK into /BAK subfolder
.. adjust upgrade procedure to copy .BAK files to /BAK
.. do not rename files in /import or /download when importing again from those dirs and the file already exists as imported file