Fix A/B test initialization

[zackkrida]

Last week I wrote the following comment:

https://github.com/creativecommons/cccatalog-frontend/blob/develop/src/components/HeaderSection.vue#L44-L47

  /* Note: this isn't the ideal place to do this, but we need this to run globally,
   * client-side, on any initial page visit,and the header is included on every
   * page so it's an okay place to do it.
   */

Sadly, I lied. This code broke A/B test initialization. I did a little research into the Nuxt lifecycle and found that the best way to run client-side code on initialization is in a custom plugin that runs client-side only. This PR implements that plugin and fixes the broken A/B test initialization.

Additionally, I found an incompatibility with Nuxt's modern output and the sixpack-client library. That library was relying on some global state (a counter variable) and had a few issues preventing it from working correctly. It also contains code to set/get cookies and a uuid, things we were already doing for our a/b test sessions.

I created a simpler ~utils/sixpack.js file with two methods that return promises: participate and convert. The participate promise will always resolve, which makes setting the experiments much simpler.

Finally, the api for an ab test is now just an object. Behold:

export default {
   name: 'experiment_name',
   defaultCase: 'experiment_default_case_name',
   cases: {
     'experiment_default_case_name',
     'any_other_case_names',
     'any_other_case_name_2',
   },
   traffic_fraction: .10 // an optional number between 0 and 1 (100%)
 }

This makes defining A/B tests much simpler, instead of repeating joinExperiment function logic every single time.

Checklist

- [x] My pull request has a descriptive title (not a vague title like `Update index.md`). - [x] My pull request targets the *default* branch of the repository (`main` or `master`). - [x] My commit messages follow [best practices][best_practices]. - [x] My code follows the established code style of the repository. - [ ] I added tests for the changes I made (if applicable). - [x] I added or updated documentation (if applicable). - [x] I tried running the project locally and verified that there are no visible errors. [best_practices]:https://gist.github.com/robertpainsi/b632364184e70900af4ab688decf6f53 ## Developer Certificate of Origin

Developer Certificate of Origin

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