Make CheckRegistry.register() idempotent.

I can't think of a good reason why it should be a problem if a check is
registered twice, but we are not going to want it to run twice and print
the errors twice.
Give contenttypes a default_app_config.
Remove the deprecation warnings, and make tests cleaner.

For now at least.
Make AdminConfig the default.

This is backwards incompatible if you do not normally use autodiscovery
and autodiscovery being called causes you errors. As far as I can tell,
if you do use autodiscovery autodiscover() being called twice does not
result in errors.
Add a note about default_app_config for app authors.