Application prefs section (#2758)

* Add code for creating/managing apps to settings section

* Add specs for app changes

* Fix controller spec

* Fix view file I pasted over by mistake

* Add locale strings. Add 'my apps' to nav

* Add Client ID/Secret to App page. Add some visual separation

* Fix rubocop warnings

* Fix embarrassing typo

I lost an `end` statement while fixing a merge conflict.

* Add code for creating/managing apps to settings section

- Add specs for app changes
- Add locale strings. Add 'my apps' to nav
- Add Client ID/Secret to App page. Add some visual separation
- Fix some bugs/warnings

* Update to match code standards

* Trigger notification

* Add warning about not sharing API secrets

* Tweak spec a bit

* Cleanup fixture creation by using let!

* Remove unused key

* Add foreign key for application<->user

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

@@ -0,0 +1,9 @@
+- content_for :page_title do
+  = t('doorkeeper.applications.new.title')
+  
+.form-container
+  = simple_form_for @application, url: settings_applications_path do |f|
+    = render 'fields', f:f
+    
+    .actions
+      = f.button :button, t('.create'), type: :submit