It was possible to remove a map location from a different proposal (even
one created by a different author) by modifying the hidden `id`
parameter in the form.
So we're making sure the map location we destroy is the one associated
to the proposal we're updating.
Since we're now using the `@proposal` instance variable in the
`destroy_map_location_association` method, we're calling that method
after loading the resource with cancancan.
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