db: introduce migration step after 93834966ae01 dropped non-nullable inherit_default_permissions

The database migration step was lazily and naively skipped ... but that turns
out to be a problem when new users are added.
In the database, the original column 'inherit_default_permissions' was
marked as non-nullable without default value. In the Kallithea code after
commit 93834966ae01, the column 'inherit_default_permissions' was no longer
known, and thus not given a value when new users are added. As a result, the
database complained:

IntegrityError: (psycopg2.errors.NotNullViolation) null value in column "inherit_default_permissions" violates not-null constraint

Fix that now by adding an appropriate db migration step to actually remove
the columns.

Use meta reflection to check if columns exist before running the upgrade
step. The upgrade step only has to be run if it is an old database - not if
it has been created after the schema changes were introduced.

For the downgrade step, make sure to set a default value for non-nullable
columns.