FIX viewmode being hard-coded making custom addons of type TAB impossible

[ndelangen]

Issue:
I was documenting the TAB addon type, and discovered we had the info and story viewmodes hardcoded, this meant it was pretty much impossible to create custom viewmodes & thus addons of type TAB.

This PR changes the way we detect whether we're in /settings/xyz.

Now addons can easily create their own TAB! no hassle.

What I did

FIX router & desktop layout, which made it impossible to add custom viewmodes

[vercel]

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[codecov]

Codecov Report

Merging #6551 into next will not change coverage.
The diff coverage is 50%.

@@           Coverage Diff           @@
##             next    #6551   +/-   ##
=======================================
  Coverage   40.77%   40.77%           
=======================================
  Files         616      616           
  Lines        8530     8530           
  Branches      596      596           
=======================================
  Hits         3478     3478           
  Misses       4960     4960           
  Partials       92       92

Impacted Files	Coverage Δ
lib/ui/src/components/layout/layout.stories.js	82.35% <ø> (ø)	⬆️
lib/ui/src/components/layout/desktop.js	100% <ø> (ø)	⬆️
lib/router/src/utils.ts	51.61% <50%> (ø)	⬆️

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[shilman]

LGTM!