Proxy letter avatars by default

On sites that don't otherwise configure an avatar fallback, Discourse will
now tell the client to get its letter avatars from a location which nginx
proxies to the centralised `avatars.discourse.org` service.  This alleviates
privacy concerns, whilst still providing some degree of performance benefit
(no need for every site to delay avatar response by 300ms for image
rendering).

It is still possible to gain the benefits of global image caching and the
lower latency of requesting directly from a CDN, by explicitly changing the
`external_system_avatars_url` site setting to
`https://avatars.discourse.org/letter/{first_letter}/{color}/{size}.png`.

app/models/user.rb

@@ -475,6 +475,7 @@ class User < ActiveRecord::Base
       url.gsub! "{color}", letter_avatar_color(username.downcase)
       url.gsub! "{username}", username
       url.gsub! "{first_letter}", username[0].downcase
+      url.gsub! "{hostname}", RailsMultisite::ConnectionManagement.current_hostname
       url
     else
       "#{Discourse.base_uri}/letter_avatar/#{username.downcase}/{size}/#{LetterAvatar.version}.png"

config/nginx.sample.conf

@@ -192,6 +192,11 @@ server {
       break;
     }
 
+    location /letter_avatar_proxy {
+      rewrite /letter_avatar_proxy/(.*)$ /$1 break;
+      proxy_pass https://avatars.discourse.org;
+    }
+
     # this means every file in public is tried first
     try_files $uri @discourse;
   }

config/site_settings.yml

@@ -593,9 +593,9 @@ files:
     client: true
     shadowed_by_global: true
   external_system_avatars_url:
-    default: "https://avatars.discourse.org/v2/letter/{first_letter}/{color}/{size}.png"
+    default: "//{hostname}/letter_avatar_proxy/v2/letter/{first_letter}/{color}/{size}.png"
     client: true
-    regex: '^https?:\/\/.+[^\/]'
+    regex: '^((https?:)?\/)?\/.+[^\/]'
   default_opengraph_image_url: ''
 
 trust:

spec/models/user_spec.rb

@@ -912,7 +912,7 @@ describe User do
       expect(user.small_avatar_url).to eq("//test.localhost/letter_avatar/sam/45/#{LetterAvatar.version}.png")
 
       SiteSetting.external_system_avatars_enabled = true
-      expect(user.small_avatar_url).to eq("https://avatars.discourse.org/v2/letter/s/5f9b8f/45.png")
+      expect(user.small_avatar_url).to eq("//test.localhost/letter_avatar_proxy/v2/letter/s/5f9b8f/45.png")
     end
 
   end