When fetching a list resource, the `serialize_object_list` was calling `serialize_object` for each item in the list but passing the request as a positional argument instead of a keyword argument. This works fine for just about everything, but the current implementation of Review Board's `MarkdownFieldsMixin` attempts to pull the request out of a kwargs dict, and so it was failing to find it, resulting in a serialized payload that did not include any of the requested text types. The result of this was that the review dialog was not able to access the raw (or even rendered) text for comments, and so clicking edit on a comment would open up a blank editor. This bug seems likely to have been exposed when we changed the inline editors in the review dialog to use the new (non-jquery-ui) implementation for inline editors, which also streamlined the way we fetch the comment data.o Ideally we should fix up every version of `serialize_object` to enforce that the request parameter is a kwarg, but that's a much larger change across multiple codebases, so I wanted to fix the bug in isolation first. Testing Done: Opened the review dialog with an existing review and was able to edit comments properly.