Issue #3638107: Allow use on admin pages

Andrew Mortonrequested to merge
issue/theme_flip-3638107:3638107-allow-use-on into 1.0.x

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Adds an admin_routes setting, off by default. While it is on, the negotiator applies on admin routes too, so the previewed theme replaces the admin theme there, and the widget shows on admin pages and on pages rendered in the admin theme.

Closes #3638107

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