Models: Update MCP models to match SQL

refs #7303
connect_and_remove_transfer_files: delete full transfer from ES

Previously, this function queried the database for a list of files in
the transfer, and removed those specifically. However, since the
search index contains many files that aren't in the database (such
as logs and metadata files), this left dangling files that could
never be removed.

Instead query the database for transfers, then delete all files in ES
from each of those transfers.

refs #7357
use generic version of elasticSearch remove files method

refs #7357

When deleting files from the ElasticSearch index, take
advantage of the change in commit c0eb597c630a482ffcb8b2056b04f3f605265114
that provides a more generic remove method.
MCPServer: replace databaseInterface with django models
Unit classes: move some methods into superclass
wip
jobChainLink: fix description
jobChainLink: fix uuid field name
archivematicaMCP: description is on type, not watched dir
Fix uuid vs id usage
jobchainlink: correct class name
linkTaskManagerChoice: id vs pk
linkTaskManagerChoice: fix using wrong variable
archivematicaMCP: use .exists()
jobchainlink: catch multiple objects returned
linkTaskManagerChoice: fix choice creation
linkTaskManagerChoice: fix variable names
linkTaskManagerChoice: look up description properly
unit: import needed models
jobChainLink: handle no MSCLs found