MCPServer: replace databaseInterface with django models
Unit classes: move some methods into superclass

Also switch from databaseInterface to Django models.

refs #7137.
databaseFunctions: use Django models

refs #7137
fileOperations: use Django models

refs #7137
elasticSearchFunctions: use Django models

refs #7137
storageService: use Django models

refs #7137
MCPClient: use Django models

refs #7137
jobChainLink: print to stderr, don't print stderr
localDevSetup: replace databaseInterface with Django models
identifyFileFormat: file_.sip won't raise
createSIPfromTransferObjects: fix subsequent runs

If this script runs more than once, the SIP objects directory will
already be populated with some files. Make sure the contents of
objects/ is a directory before trying to list its contents, otherwise
this will fail.
dicts: don't try to expand path if sipdir not defined
insertIntoDerivations: source and derived files are mandatory
databaseFunctions: add docstrings
databaseFunctions: add tests
main.models: add FilesIDs model
Move getUTCDate, getDeciDate to databaseFunctions

refs #7137
databaseInterface: remove separator
views_atk: remove unused import

refs #4757
archivematicaMoveSIP: remove unused imports