XBEndpointRenderer adds response headers that sometimes exceed common server limits

Overview

XBEndpointRenderer adds Attach-Css, Attach-Js, and Attach-Settings response headers. While the HTTP standard doesn't set a limit for response header sizes, common web servers do; for example, by default, Apache limits to 8KB and Nginx limits to 4KB. In some cases, these Attach-* values can be larger than that; for example, see #3475652-24: Should we add SDDS to XB's Tugboat?.

Proposed resolution

While one could come up with ideas for how to work around the header size limit; for example, splitting large values into multiple headers, or trying to compress the header values, or when the value exceeds a certain size, replace it with a URL that the client can use to fetch the full header value, I think the most straightforward solution is to move these values into the response body.

In other words, change the response body from HTML to JSON, where the keys of the JSON object can be attachments and content.

User interface changes