Post-Installation Messages extension drop-down

Please review the discussion in issue #4141 and reply here.

This PR implements an extension selection drop-down field in the post-installation messages view.

Moreover, this PR fixes the issues with the wrong extension ID being used when you are resetting the post-installation messages.

Finally, it adds a model method to add post-installation messages in the database. The upside of using it instead of directly inserting entries in the database is that it gives you intelligible error messages if you screw up :)

Close gh-4141 (hopefully this will close the other issue once we merge this PR)

@Bakual @n3t Please review this PR since you were involved in the original issue. Thanks!