db: enable use of main Kallithea config as Alembic config

Newly generated Kallithea config .ini files will be valid Alembic
config files, eliminating the need for a separate alembic.ini config
redundantly specifying the database connection string.

We reference the Alembic migration environment using kallithea:alembic,
which should work independently of how Kallithea is installed.

We also configure a default 'alembic' log level of WARNING, to reduce
the amount of clutter in the config file, reduce the changes needed
to upgrade existing config files for use with Alembic, and allowing us
to change the default Alembic log level for all users down the road.

(It makes sense to define Alembic logging in code, while all other
loggers are configured in the configuration file, because Alembic
is special: it runs on the command line, not as part of the web app.)