paster: add informational/dummy "upgrade-db" command

The command is placed kallithea.lib.dbmigrate:UpgradeDb (which was the
location of the old command, too), to ensure that "paster upgrade-db"
continues to work, even if Kallithea is installed in "editable" mode
(setup.py develop/pip install -e) and package metadata has not been
updated (and also to prevent issues caused by stale .pyc files).