Update error messages for letters that are too long

To specify the number of sheets as well as pages in case this is not
clear.

app/templates/partials/check/letter-too-long.html

@@ -2,7 +2,7 @@
   Your letter is too long
 </h1>
 <p>
-  Letters must be {{ letter_max_pages }} pages or less.
+  Letters must be {{ letter_max_pages }} pages or less ({{ letter_max_pages // 2 }} double-sided sheets of paper).
   <br>
   Your letter is {{ page_count }} pages long.
 </p>

app/templates/views/guidance/upload-a-letter.html

@@ -29,7 +29,7 @@
   <ul class="list list-bullet">
     <li>a PDF</li>
     <li>A4 portrait size (210 × 297 mm)</li>
-    <li>10 pages or less</li>
+    <li>10 pages or less (5 double-sided sheets of paper)</li>
     <li>smaller than 2MB</li>
   </ul>
 

app/templates/views/uploads/choose-file.html

@@ -44,7 +44,7 @@
     <ul class="list list-bullet">
       <li>a PDF</li>
       <li>A4 portrait size (210 × 297 mm)</li>
-      <li>10 pages or less</li>
+      <li>10 pages or less (5 double-sided sheets of paper)</li>
       <li>smaller than 2MB</li>
     </ul>