gui: Set CSRF stuff earlier (fixes #3138)

We need to set these properties *before* Angular starts making requests,
and doing that from the response to a request is too late. The obvious
choice (to me) would be to use the angular $cookies service, but that
service isn't available until after initialization so we can't use it.
Instead, add a special file that is loaded by index.html and includes
the info we need before the JS app even starts running.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3152

gui/default/index.html

@@ -664,6 +664,7 @@
 
   <script src="assets/lang/valid-langs.js"></script>
   <script src="assets/lang/prettyprint.js"></script>
+  <script src="meta.js"></script>
   <script src="syncthing/app.js"></script>
   <!-- / gui application code -->