Add display and form for existing pages_read

Commented out the new update type because it was breaking and I don't
need it quite yet

bookwyrm/view_actions.py

@@ -729,4 +729,12 @@ def update_readthrough(request, book=None, create=True):
         except ParserError:
             pass
 
+    pages_read = request.POST.get('pages_read')
+    if pages_read:
+        try:
+            pages_read = int(pages_read)
+            readthrough.pages_read = pages_read
+        except ValueError:
+            pass
+
     return readthrough