Simplify validation of saved basket

Before this commit, all products of Lines in the saved basket would be
checked for availability on each action, even on unrelated actions
like deleting a line or moving another line to the basket. This was
mentioned in issue #666.
This changeset only does the availability check if the Line is to be
moved back to the basket.

I refactored the fields list for SavedLineForm from blacklisting six
fields to whitelisting two field for clarity reasons.

Fixes #666