Add ID-based pagination to logs using new decorators and an encrypted token

Fixes #599

data/model/modelutil.py

@@ -0,0 +1,29 @@
+def paginate(query, model, descending=False, page_token=None, limit=50):
+  """ Paginates the given query using an ID range, starting at the optional page_token.
+      Returns a *list* of matching results along with an unencrypted page_token for the
+      next page, if any. If descending is set to True, orders by the ID descending rather
+      than ascending.
+  """
+  query = query.limit(limit + 1)
+
+  if descending:
+    query = query.order_by(model.id.desc())
+  else:
+    query = query.order_by(model.id)
+
+  if page_token is not None:
+    start_id = page_token.get('start_id')
+    if start_id is not None:
+      if descending:
+        query = query.where(model.id <= start_id)
+      else:
+        query = query.where(model.id >= start_id)
+
+  results = list(query)
+  page_token = None
+  if len(results) > limit:
+    page_token = {
+      'start_id': results[limit].id
+    }
+
+  return results[0:limit], page_token