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.

app/logical/custom_css.rb

@@ -0,0 +1,15 @@
+class CustomCss
+  attr_reader :css
+
+  def initialize(css)
+    @css = css
+  end
+
+  def valid?
+    css.blank? || parsed_css.none? { |node| node[:node] == :error }
+  end
+
+  def parsed_css
+    @parsed_css ||= Crass.parse(css, preserve_comments: true)
+  end
+end