Redirect sign-in and register links to a beta landing page

Public registrations on app.happydomain.org are paused. Route the
sign-in, register, and join entry points to a new /beta/ page that
explains the beta program and points users to self-hosting via the
binary or Docker image.

layouts/partials/cta-join.html

@@ -13,7 +13,7 @@
                 </p>
             </div>
             <a
-                href="/register"
+                href="/beta/"
                 class="btn btn-lg btn-light px-4"
                 style="border-radius: 2em"
                 data-umami-event="cta-join"