Fix #4111: Cache custom CSS.

app/logical/custom_css.rb

@@ -0,0 +1,11 @@
+module CustomCss
+  def self.parse(css)
+    css.to_s.split(/\r\n|\r|\n/).map do |line|
+      if line =~ /\A@import/
+        line
+      else
+        line.gsub(/([^[:space:]])[[:space:]]*(?:!important)?[[:space:]]*(;|})/, "\\1 !important\\2")
+      end
+    end.join("\n")
+  end
+end