Improve avatar image handling

- redirect avatar image requests to the rails_blob_url, instead of
  streaming them through the web app
- use a thumbnail variant for avatar images
- only put avatar initials behind the stale? check (not the image
  redirect, which would result in browsers rendering broken images when
  an avatar is changed, until max-age expires)

test/controllers/users/avatars_controller_test.rb

@@ -5,19 +5,37 @@ class Users::AvatarsControllerTest < ActionDispatch::IntegrationTest
     sign_in_as :david
   end
 
-  test "show self without caching" do
+  test "show own initials without caching" do
     get user_avatar_path(users(:david))
     assert_match "image/svg+xml", @response.content_type
     assert @response.cache_control[:private]
     assert_equal "0", @response.cache_control[:max_age]
   end
 
-  test "show other with caching" do
+  test "show other initials with caching" do
     get user_avatar_path(users(:kevin))
     assert_match "image/svg+xml", @response.content_type
     assert_equal 30.minutes.to_s, @response.cache_control[:max_age]
   end
 
+  test "show own image redirects to the blob url" do
+    users(:david).avatar.attach(io: File.open(file_fixture("moon.jpg")), filename: "moon.jpg", content_type: "image/jpeg")
+    assert users(:david).avatar.attached?
+
+    get user_avatar_path(users(:david))
+
+    assert_redirected_to rails_blob_url(users(:david).avatar.variant(:thumb), disposition: "inline")
+  end
+
+  test "show other image redirects to the blob url" do
+    users(:kevin).avatar.attach(io: File.open(file_fixture("moon.jpg")), filename: "moon.jpg", content_type: "image/jpeg")
+    assert users(:kevin).avatar.attached?
+
+    get user_avatar_path(users(:kevin))
+
+    assert_redirected_to rails_blob_url(users(:kevin).avatar.variant(:thumb), disposition: "inline")
+  end
+
   test "delete self" do
     delete user_avatar_path(users(:david))
     assert_redirected_to users(:david)