Adds a migration that initialises a legacy tokenizer for
an existing database. The migration is not active yet as
it will need completion when more functionality is added
to the legacy tokenizer.
This adds the boilerplate for selecting configurable tokenizers.
A tokenizer can be chosen at import time and will then install
itself such that it is fixed for the given database import even
when the software itself is updated.
The legacy tokenizer implements Nominatim's traditional algorithms.