[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