Upgrade Rails to version 5.2.1 and adopt code & specs where necessary

Signed-off-by: Sebastian Serth <Sebastian.Serth@student.hpi.de>

app/assets/javascripts/application.js

@@ -11,16 +11,23 @@
 // about supported directives.
 //
 //= require jquery
-//
-//= require ace/ace
-//= require chosen.jquery.min
-//= require jquery-ui.min
-//= require d3
 //= require jquery.turbolinks
 //= require jquery_ujs
-//= require jstree/jstree.min
-//= require turbolinks
-//= require_tree ../../../lib
-//= require_tree .
 //= require bootstrap_pagedown
-//= require ace/ext-language_tools
+//= require d3
+//= require turbolinks
+//
+// lib/assets
+//= require flash
+//= require url
+//
+// vendor/assets
+//= require ace/ace
+//= require ace/ext-language_tools
+//= require jstree/jstree.min
+//= require bootstrap.min
+//= require chosen.jquery.min
+//= require jquery-ui.min
+//= require underscore.min
+//= require vis.min
+//= require_tree .

app/assets/javascripts/dashboard.js

@@ -51,6 +51,7 @@ $(function() {
     } else {
       var jqxhr = $.ajax({
         dataType: 'json',
+        url: 'dashboard',
         method: 'GET'
       });
       jqxhr.done(function(response) {

app/assets/stylesheets/application.css

@@ -10,9 +10,19 @@
  * defined in the other CSS/SCSS files in this directory. It is generally better to create a new
  * file per style scope.
  *
- *= require_tree .
- *= require_tree ../../../lib
- *= require_tree ../../../vendor/assets/stylesheets/
- *= require_self
  *= require bootstrap_pagedown
+ *
+ * lib/assets
+ *= require flash
+ *
+ * vendor/assets
+ *= require bootstrap.min
+ *= require chosen.min
+ *= require font-awesome.min
+ *= require google_opensans
+ *= require jquery-ui.min
+ *= require jquery-ui.structure.min
+ *= require style.min
+ *= require vis.min
+ *= require_tree .
 */