Uploads for admin site settings (#4913)

* Improve OpenGraph tags for about pages

* Add thumbnail admin setting

* Fix error

* Fix up

config/locales/en.yml

@@ -195,6 +195,9 @@ en:
         desc_html: You can write your own privacy policy, terms of service or other legalese. You can use HTML tags
         title: Custom terms of service
       site_title: Instance name
+      thumbnail:
+        desc_html: Used for previews via OpenGraph and API. 1200x630px recommended
+        title: Instance thumbnail
       timeline_preview:
         desc_html: Display public timeline on landing page
         title: Timeline preview