When a Group is missing a route, the migration failed previously with a
`Invalid single-table inheritance type` error. To fix this, we can
disable STI for the migration class because we don't need to know about
the type to do this migration. Besides, currently Group is the only type
used in the type column.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58714
This adds a database migration that creates routes for any projects and
namespaces that don't already have one. We also remove the runtime code
for dynamically creating routes, as this is no longer necessary.