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"

layouts/partials/jumbo.html

@@ -155,7 +155,7 @@
                         <i class="bi bi-info-circle"></i>
                         {{ i18n "beta-alert-title" }}
                     </h5>
-                    <p class="mb-0">{{ i18n "beta-alert-text" }}</p>
+                    <p class="mb-0">{{ i18n "beta-alert-text" | safeHTML }}</p>
                 </div>
                 <form
                     class="col-lg-5 d-flex flex-column flex-sm-row gap-2"