[frontend] don't unselect inactive chroots in project edit; fix #712
    
    When editting project settings, the chroots are processed in the
    following matter. First, we figure out what chroots are currently
    enabled (this list would contain even inactive chroots), then we
    figure out what chroots has been selected in the submitted form.
    And finally, we add selected chroots that aren't currently enabled
    and ... remove currently enabled chroots, that are not in the
    submitted form (the form doesn't contain inactive chroots, so
    obviously, they disappear and cause #712).
    
    Merges: PR#719