fix(storybook): use plex in manager ui even when not installed

[tay1orjones]

Something I noticed in #16910 - on machines that do not have Plex installed, the storybook uses a system default font.

Changelog

Changed

• Update storybook's manager-head.html to load plex from the CDN into the Manager portion of storybook (sidebar nav, controls pane, etc.)

Testing / Reviewing

• Open the deploy preview on a machine with plex installed
  • inspect in the firefox dev tools 'fonts' pane, the locally installed version of plex should be on the page in addition to the one(s) loaded from the deploy preview itself.
• Open the deploy preview on a machine without plex installed
  • inspect in the firefox dev tools ... you should see plex being loaded from s81c.com/...
  • The storybook Manager ui should be rendered with plex

Windows w/out local Plex (old)	Windows w/out local Plex (this PR)	Mac w/ local Plex (prod)

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[emyarod]

looks good to me!