HTTP Security plug: make starting csp string generation more readable

lib/pleroma/plugs/http_security_plug.ex

@@ -49,7 +49,7 @@ defp headers do
     end
   end
 
-  @csp_start [
+  static_csp_rules = [
     "default-src 'none'",
     "base-uri 'self'",
     "frame-ancestors 'none'",
@@ -57,9 +57,8 @@ defp headers do
     "font-src 'self'",
     "manifest-src 'self'"
   ]
-             |> Enum.join(";")
-             |> Kernel.<>(";")
-             |> List.wrap()
+
+  @csp_start [Enum.join(static_csp_rules, ";") <> ";"]
 
   defp csp_string do
     scheme = Config.get([Pleroma.Web.Endpoint, :url])[:scheme]