[FIX] web: basic_controller: wait for the mutex before discard changes

Before this commit, the discard confirm dialog was directly shown
without waiting for the write RPC. For instance, it the user clicked on 'Save'
after editing a record, and then directly clicked on another menu or on the
breadcrumb before the write RPC returned, the record was still considered
as dirty and thus the confirm dialog was displayed. We now wait for the RPC
to return before checking if the record is dirty, so that the confirm dialog
is only displayed when the record is actually dirty.