Fix #4678: Validate custom CSS.

* Make it an error to add invalid custom CSS to your account.
* Add a fix script to remove custom CSS from all accounts with invalid CSS.

script/fixes/091_delete_invalid_custom_css.rb

@@ -0,0 +1,21 @@
+#!/usr/bin/env ruby
+
+require_relative "base"
+
+with_confirmation do
+  users = User.where("custom_style != ''").select { |user| !user.custom_css.valid? }
+
+  users.each do |user|
+    Dmail.create_automated(to: user, title: "Action required: Your custom CSS is invalid", body: <<~EOS)
+      Hi,
+
+      The custom CSS in your account settings is invalid and has been removed from your account. To restore it, go to https://codebeautify.org/cssvalidate, copy and paste the CSS below, and fix any errors that are shown. Then, go to your account settings at https://danbooru.donmai.us/settings, click the Advanced tab, and re-add your custom CSS.
+
+      [code]
+      #{user.custom_style}
+      [/code]
+    EOS
+
+    user.update!(custom_style: "")
+  end
+end