Make sure the migration steps have been done before running Commerce Manager

Steps to reproduce

If the customer start Commerce Manager before the migration steps are completed, they might run into issues (For example on 8.16.1, they won't be able to log in to CM). We should enforce customer to complete the migration steps before we run CM.

One possible approach:

- In MigrationManager, if there are no content models (ApplicationContext.HasContentModelTypes), and there are incompleted migration steps, throw error to let user know migration step should be run in fron end site, then restart Commerce Manager.

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How to test:

- Install a site from VS, upgrade it to latest EpiServer.Commerce version

- Add another site to solution, upgrade it to latest EPiServer.Commercemanager version

- building site, update database etc, then browse to commerce manager site first

Expected: It should throws error that the migration steps are not completed. After completing steps in front-end site and restart CM site, it should be accessible as normal