Do not hide modal on back button if Turbolinks enabled

There doesn't seem to be any way to prevent Turbolinks from reloading
and replacing the entire browser window when a user clicks Back (since
turbolinks:before-visit is not triggered for _when navigating by
history_),

So, rather than hiding the displayed modal when clicking Back (and
then having to reload the page), we keep the Modal displayed while
Turbolinks does its stuff in the background (including progress bar).

See turbolinks/turbolinks#264 for a related
issue (suggested `turbolinks:before-history-change` event).

jquery.modal.js

@@ -85,7 +85,13 @@
         // popstate gets the _after_ state, not the _before_ state
         $.modal.handlePopstate = function(event) {
           if (event.state.no_modals) {
-            $.modal.close();
+            if (typeof Turbolinks != 'undefined' && Turbolinks.supported) {
+              // do nothing; Turbolinks will be reloading the page anyway, unfortunately,
+              // so rather than hiding a modal (suggesting the page is ready) and then refreshing
+              // the page, we keep the modal displayed while the page reloads
+            } else {
+              $.modal.close();
+            }
           } else if (event.state.modal) {
             elm = $(event.state.modal);
             elm.trigger($.modal.REOPEN, event);