Add 'New' tag to legal mechanism pages (#507)

This aims to acknowledge that this is a newly-added question, whilst not
drawing too much attention to the feature (as we think it is largely
self-explanatory).

We’d remove the tag after a month or 2.

## Screenshots

### When recording a vaccination

<img width="858" height="572" alt="Screenshot 2025-10-27 at 09 47 52"
src="https://github.com/user-attachments/assets/b2a5796d-20dc-42f3-b418-b9f326de1a36"
/>

### When adding a new vaccine to a site

<img width="976" height="621" alt="Screenshot 2025-10-27 at 09 48 56"
src="https://github.com/user-attachments/assets/5f552092-c6e5-4c96-8901-fd5115d34292"
/>

Author: frankieroberto

app/views/record-vaccinations/legal-mechanism.html

@@ -21,6 +21,11 @@
 
       <form action="/record-vaccinations/answer-legal-mechanism" method="post" novalidate>
 
+        {{ tag({
+          text: "New",
+          classes: "nhsuk-tag nhsuk-tag--blue nhsuk-u-margin-bottom-3"
+        }) }}
+
         {{ radios({
           idPrefix: "legal-mechanism",
           name: "legalMechanism",

app/views/vaccines/legal-mechanisms.html

@@ -16,6 +16,11 @@
     <div class="nhsuk-grid-column-two-thirds">
       <form action="/vaccines/check" method="post" novalidate="true">
 
+        {{ tag({
+          text: "New",
+          classes: "nhsuk-tag nhsuk-tag--blue nhsuk-u-margin-bottom-3"
+        }) }}
+
         {{ checkboxes({
           idPrefix: "legal-mechanisms",
           name: "vaccineLegalMechanisms",