js: fixup issues with @rails/ujs.

Fix a couple regressions caused by the migration from jquery-ujs to
@rails/ujs in 9f4ac4c96:

* Add authenticity tokens to all remote forms. By default, Rails doesn't
  doesn't include authenticity tokens in remote forms because it can cause
  problems with fragment caching. This was fine with jquery-ujs because it
  would insert the authenticity token when the remote form was
  submitted, but apparently @rails/ujs doesn't do this. This broke
  certain remote forms nested inside of jquery UI dialogs.

* Fix dialogs to trigger remote form submissions through @rails/ujs
  instead of through jquery. This fixes a problem where remote forms that
  returned a javascript response displayed the response as plaintext
  instead of executing the returned javascript.

app/javascript/src/javascripts/artist_commentaries.js

@@ -1,4 +1,5 @@
 import Utility from "./utility";
+import Rails from "@rails/ujs";
 
 let ArtistCommentary = {};
 
@@ -34,7 +35,8 @@ ArtistCommentary.initialize_edit_commentary_dialog = function() {
     width: 700,
     buttons: {
       "Submit": function() {
-        $("#add-commentary-dialog #edit-commentary").submit();
+        let form = $("#add-commentary-dialog #edit-commentary").get(0);
+        Rails.fire(form, "submit");
         $(this).dialog("close");
       },
       "Cancel": function() {

app/javascript/src/javascripts/posts.js.erb

@@ -3,6 +3,7 @@ import Utility from './utility'
 import Hammer from 'hammerjs'
 import Cookie from './cookie'
 import Note from './notes'
+import Rails from '@rails/ujs'
 
 let Post = {};
 
@@ -556,7 +557,8 @@ Post.initialize_saved_searches = function() {
     autoOpen: false,
     buttons: {
       "Submit": function() {
-        $("#save-search-dialog form").submit();
+        let form = $("#save-search-dialog form").get(0);
+        Rails.fire(form, "submit");
         $(this).dialog("close");
       },
       "Cancel": function() {

app/javascript/src/javascripts/utility.js

@@ -1,4 +1,5 @@
 import CurrentUser from "./current_user";
+import Rails from '@rails/ujs';
 
 let Utility = {};
 
@@ -54,7 +55,8 @@ Utility.dialog = function(title, html) {
     },
     buttons: {
       "Submit": function() {
-        $dialog.find("form").submit();
+        let form = $dialog.find("form").get(0);
+        Rails.fire(form, "submit");
       },
       "Cancel": function() {
         $dialog.dialog("close");