Show all pages of a letter everywhere

Right now showing all the pages in full is the only way we have of
showing a letter that makes sense to our users. Maybe in the future we
show some kind of truncated version, but the end of the first page is
not a good place to truncate the letter.

This commit just extracts the code for showing multiple pages from the
template view, refactors it for reuse, and includes it in the send
views.

app/template_previews.py

@@ -1,4 +1,4 @@
-from flask import current_app
+from flask import current_app, json
 import requests
 
 from app import current_service
@@ -32,3 +32,14 @@ class TemplatePreview:
             template.values,
             page=page,
         )
+
+
+def get_page_count_for_letter(template, values=None):
+
+    if template['template_type'] != 'letter':
+        return None
+
+    page_count, _, _ = TemplatePreview.from_database_object(template, values, filetype='json')
+    page_count = json.loads(page_count.decode('utf-8'))['count']
+
+    return page_count