Move all the functions to a particular module in the utils folder,
avoiding circular imports and having a more clear view of the functions

The images for the tenprint template are empty, hence not encrypted in
the database. This patch add the selective decryption for images (only
submission images are decrypted)

This commit add the functionnality to recreate the DEK based upon the
informations known by the submitter, even if the donor has removed his
DEK from the database. This DEK is only stored in the session of the
submitter, and is deleted once logged out.