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`.

config/initializers/vips.rb

@@ -0,0 +1,9 @@
+# Disable unfuzzed libvips operations.
+#
+# To block loaders we need to call `Vips.block` after Rails and image_processing set their
+# defaults. Force the order of operations by autoloading the file now.
+ActiveStorage::Transformers::Vips
+Vips.block_untrusted(true)
+Vips.block("VipsForeignLoadOpenslide", true) # prevent sqlite segfault in forked parallel workers
+Rails.application.config.active_storage.variable_content_types -=
+    %w[ image/bmp image/vnd.microsoft.icon image/vnd.adobe.photoshop ]