Add option to copy existing template when adding

Sometimes when setting up a service you might have a few very similar
templates, in which only a small amount of content. Or you might even
have a few of services, which are used by different teams but have
similar templates.

Copy and pasting, especially from one service to another, is a pain.
This commit makes it easier by allowing users to copy an existing
template when choosing to add a new one, instead of starting from
scratch.

app/templates/views/edit-sms-template.html

@@ -13,7 +13,10 @@
       {{ heading_action }} text message template
     </h1>
 
-    <form method="post">
+    <form
+      method="post"
+      action="{{ url_for('.add_service_template', service_id=current_service.id, template_type=template_type) }}"
+    >
       <div class="grid-row">
         <div class="column-two-thirds">
           {{ textbox(form.name, width='1-1', hint='Your recipients won’t see this') }}