Plugs.HTTPSecurityPlug: Add static_url to CSP's connect-src

Closes: !469

lib/pleroma/plugs/http_security_plug.ex

@@ -34,13 +34,16 @@ defp headers do
 
   defp csp_string do
     scheme = Config.get([Pleroma.Web.Endpoint, :url])[:scheme]
-    websocket_url = String.replace(Pleroma.Web.Endpoint.static_url(), "http", "ws")
+    static_url = Pleroma.Web.Endpoint.static_url()
+    websocket_url = String.replace(static_url, "http", "ws")
+
+    connect_src = "connect-src 'self' #{static_url} #{websocket_url}"
 
     connect_src =
       if Mix.env() == :dev do
-        "connect-src 'self' http://localhost:3035/ " <> websocket_url
+        connect_src <> " http://localhost:3035/"
       else
-        "connect-src 'self' " <> websocket_url
+        connect_src
       end
 
     script_src =