Importing needs validation

[nerocui]

Currently importing depends on the excel being in the correct format. While that's still required, we should validate the values before importing them into the backend.
Let's have a validate page to show the admin what's going to be insert. And let's only insert if the admin click "confirm import".
The validate page should show a editable grid, so if any of the fields are invalid, the admin can just change them inline.