Some items could optionally be dropped from stats.json

Not all of the fields in stats.json appear to be needed.
As this is one of the largest responses, it would make sense to try and trim it down if possible.

[Note: for backwards compatibility, this should be optional]

I think the following could be dropped:
In the emails list:
- attachments: this does not appear to be needed for the display; if necessary there could be a boolean (or count) which is only present if there is at least one attachment.
- list & list_raw: are these both needed?
- ditto id and mid

In the thread_struct: unless there are child entries, is the parent needed at all?

Start by locating the code that produces stats.json and trace how the emails list and thread_struct are used for display. Check whether attachments, list, list_raw, id, mid, and parent are required by current consumers; done means optional omission is backward-compatible and reduces the response without breaking the display.

Written by the indexing model from the issue text.