Following a suggestion from :mkmelin, this seems like an optimal solution: the overriding/duplication in m-c is removed, and all users get a more powerful default choice that they're still able to override with their own, should they so wish.
For clarity and to match other `about:` pages, the shared code is placed under `toolkit/content/`, and all content under `docshell/resources/` is removed.
Differential Revision: https://phabricator.services.mozilla.com/D156478
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