This patch refactors how we check for text formats when deciding how to handle
resources, such that more text MIME types will be rendered in-browser, rather
than downloaded.
This change requires us to move more away from using the Gecko-Content-Viewers
category in the category manager for this decision, as we need to handle an
unlimited number of MIME types behind the scenes.
Support for Gecko-Content-Viewers was left in for both the in-tree use for
application/http-index-format and dynamically determining whether image/avif
and image/jxl are supported, as well as for the message/rfc822 type used by
Thunderbird.
Differential Revision: https://phabricator.services.mozilla.com/D212078
855d5761c0