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20140327 (Thursday, 27 March 2014)
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The `Migrator` class - inheritance for Migrations
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The migration which I used yesterday in Chatelet to migrate the
production server was only for the "Chatelet" variant of Lino
Welfare. That is, for a Lino Welfare which uses the "alternate"
implementation of the `courses` app.

This was enough to run the upgrade in Chatelet, which was my most
urgent goal. But I must not leave it as this since in an "Eupen"
variant they don't want to convert their `courses` tables.

Now I replaced :setting:`migration_module` by
:setting:`migration_class`.  This allows us to define "Migrators" as
classes who inherit from each other.  Technically I could have the
Site class themself, but it looks more clean when migration code is in
a single module (or package), but not together with settings.

Usage example: module :mod:`lino_welfare.settings.chatelet.migrate`.

Note: yes, Django 1.7 will have its own migrations, basically inspired
by South. That might look as a clear reason to abandon `north` and
start using Django migrations.  Note that if you don't like `north`,
you can simply use South or Django 1.7 migrations.  But:

- There is no immediate need for me to learn. I'll wait at least until
  Django 1.7 is released.

- There are certain Lino magics for which I don't yet see how I'd do
  them using South. Examples:

  - migrating from one language distribution to another
  - injected fields and interdependent apps



Welfare Uploads
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