Load testimonials by tag instead of by name

- Allow loading testimonials by tag instead of name.
- Remove the code to load testimonials by name.
- Add a `limit` and `offset` to create different lists on the same page
  - e.g. on the team coaching page where I have two testimonial
  sections.

source/_pages/atdc.md

@@ -20,14 +20,7 @@ Learn to test things like:
 
 {% block content_bottom %}
   <div class="space-y-12">
-   {% include 'testimonials.html.twig' with {
-    names: [
-      "Matthieu Scarset",
-      "Mike Karthauser",
-      "Tawny Bartlett",
-      "Scott Euser",
-      ]
-    } %}
+    {% include 'testimonials' with { tag: 'subscription' } %}
 
     {{ parent() }}
   </div>