Move Plausible integration to our standard library

[agjohnson]

With the addition of JS logic to make Plausible work, it should be defined alongside our other JS sources. Generally speaking, this is the pattern we're taking with application and website JS. The reasons for this are to remove the need for inline JS for security reasons, to push dependencies into a place they can be tracked, to maintain all JS code using linting/testing/etc tools, and to bundle everything up into a single JS artifact.

A few key differences between the JS here and the JS in #155:

• This JS works for more than just anchor elements. Using SUI, we'll want to track on buttons, divs, etc, in addition to anchor elements
• Use the Plausible library they publish on NPM instead of the include script inline
• Rely on a callback to redirect the browser
• Some cleaner code around conditionals

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:books: Documentation preview :books:: https://read-the-docs-site-community--156.com.readthedocs.build/en/156/

[agjohnson]

Looks like this failed the build check, I'll open a separate PR