Use async instead of defer for analytics script

We had this issue on Impress 2020 and I fixed it over there too. I guess
it went less noticed here on Classic, because it's a more
progressively-enhanced site in general (and this failure case is an
interesting argument for that architecture! lol).

On Impress 2020, I wasn't sure if the "waits for the document to load"
behavior of the `defer` attribute was necessary to the script, so I
chose to keep `defer` but move it _after_ the other scripts.

This time, I dug in a bit more, and found a Plausible author saying
that the choice was kinda arbitrary; and another person who had the
same issue as me, who said they switched to `async` and it worked well.
So, that's what we're doing now, too!

https://github.com/plausible/analytics/discussions/1907#discussioncomment-2754499

app/views/application/_analytics.html.erb

@@ -1,2 +1,2 @@
 <%= javascript_include_tag 'https://analytics.openneo.net/js/script.js',
-    defer: true, 'data-domain': 'impress.openneo.net' %>
+    async: true, 'data-domain': 'impress.openneo.net' %>