Disable libvips unfuzzed operations (#226)

and add test coverage for (un)supported file types.

The avatar and logo variants move into the models and return nil for content
types that are no longer variable, so the controllers fall back to the initials
avatar and stock logo icon instead of raising `ActiveStorage::InvariableError`.

test/controllers/users/avatars_controller_test.rb

@@ -18,6 +18,14 @@ class Users::AvatarsControllerTest < ActionDispatch::IntegrationTest
     assert_equal "image/webp", @response.content_type
   end
 
+  test "show initials when image cannot be resized" do
+    users(:kevin).update! avatar: fixture_file_upload("pixel.bmp", "image/bmp")
+    get user_avatar_url(users(:kevin).avatar_token)
+
+    assert_response :success
+    assert_select "text", text: "K"
+  end
+
   test "show image with invalid token responds 404" do
     get user_avatar_url("not-a-valid-token")