[FIX] web: discarding a new record in a modal closes the modal

Discarding a new record triggers the 'history_back' action.  This makes
sense if the record was discarded from a normal form view, but not so
much when in a modal, where the expected behaviour is to close the
dialog.

This commit overrides the function in the action manager which handles
the switch to the previous view. It checks whether a dialog window is
open, in which case it simply closes it.