Fix up the applications area (#4664)

- Section it into "Development" area
- Improve UI of application form, index, and details

app/views/settings/applications/new.html.haml

@@ -1,9 +1,8 @@
 - content_for :page_title do
   = t('doorkeeper.applications.new.title')
+
+= simple_form_for @application, url: settings_applications_path do |f|
+  = render 'fields', f: f
   
-.form-container
-  = simple_form_for @application, url: settings_applications_path do |f|
-    = render 'fields', f:f
-    
-    .actions
-      = f.button :button, t('.create'), type: :submit
+  .actions
+    = f.button :button, t('doorkeeper.applications.buttons.submit'), type: :submit