since [0] nova technically can allow non-admin users to do
cold migrations without specifying the target host.
This patch makes the cold migration action available in project
dashboard by moving MigrateInstance form to project dashboard.
Its availability is determined by the same policy rule,
and admin dashboard now just re-uses the from from project dashboard.
[0] https://review.opendev.org/c/openstack/nova/+/881562
Change-Id: Ic6a0c1c85cbe507b70ffdebeffb68297ca4338fe
84439db87c