[FIX] web: fix two issues when discarding records

This commit actually fixes a few problems:

1. the _visitChildren method in the basic model was not following the
changes, only the data, so it was not correct (for example, the isDirty
method was wrong for relational data, when no other change was done)

2. new records could not be discarded, because they had no data in their
data key. What we do here is to add a savePoint, so it is safe to
restore them (it caused a crash)

3. the save method was not properly following children when it was
called with the option savePoint=true

4. when the user tried to discard a form view, in an action with only a
form view, it was redirected to the previous url (via the history_back
action), instead of simply discarding the current form view