ReplacementDict: add constructor from DB data

This new constructor allows a ReplacementDict to be constructed by
passing in rows from the database. This allows places outside of the
Unit classes to gain access to the full suite of standard unit
variables without having to fully reimplement the logic.

Eventually the three Unit classes can probably be migrated to use this
constructor, ensuring that there's only one canonical way to produce
this style of ReplacementDict.

refs #6451
normalize: include standard unit variables

This uses the ReplacementDict.frommodel constructor to get access to
all the standard File unit variables in addition to the special
normalization unit variables. This provides more versatility when
writing normalization and normalization verification commands.

refs #6451
Move passClasses to archivematicaCommon

These classes have no actual dependencies on any contents of MCPServer,
and are now being used in multiple modules, so move them to
archivematicaCommon.

The module is renamed from passClasses to dicts.

Refs #6451.
ReplacementDict.replace: use doctest-compatible docstring
ReplacementDict.fromstring: add docstring