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`.

app/models/account.rb

@@ -1,6 +1,14 @@
 class Account < ApplicationRecord
   include Joinable
 
-  has_one_attached :logo
+  has_one_attached :logo do |attachable|
+    attachable.variant :large, resize_to_limit: [ 512, 512 ], format: :png
+    attachable.variant :small, resize_to_limit: [ 192, 192 ], format: :png
+  end
+
   has_json :settings, restrict_room_creation_to_administrators: false
+
+  def logo_variant(size)
+    logo.variant(size).processed if logo.variable?
+  end
 end

app/models/user/avatar.rb

@@ -2,7 +2,9 @@ module User::Avatar
   extend ActiveSupport::Concern
 
   included do
-    has_one_attached :avatar
+    has_one_attached :avatar do |attachable|
+      attachable.variant :square, resize_to_limit: [ 512, 512 ], format: :webp
+    end
   end
 
   class_methods do
@@ -14,4 +16,8 @@ module User::Avatar
   def avatar_token
     signed_id(purpose: :avatar)
   end
+
+  def avatar_variant
+    avatar.variant(:square).processed if avatar.variable?
+  end
 end