Move content-server experiment base logic into fxa-shared

[vbudhram]

While working on #2595, I didn't want to duplicate the experiment grouping logic found in the content-server to the auth-server.

I think it makes sense to pull this base class into fxa-shared and reused in both services. However, I still think the experiment rules should still live in the respective services.

┆Issue is synchronized with this Jira Task
┆Issue Number: FXA-497

[vbudhram]

As mentioned in #3183, I was not able to move over the base experiment.

If you pull down that PR and change

fxa/packages/fxa-content-server/app/scripts/lib/experiments/grouping-rules/is-sampled-user.js

Line 10 in 8701348

const BaseGroupingRule = require('./base');

to be

const BaseGroupingRule = require('../../../../../../fxa-shared/dist/experiments/base');

It should have in theory worked because it is loading the same javascript file. However, you will get an interesting error

Uncaught TypeError: BaseGroupingRule is not a constructor

when you try to load the content-sever webpage. After some searching it seems to be related to babel/babel#9367. Babel seems to fail loading the class that is outside the project causing the extends not to work correctly.

I tried updating our webpack config to ['@babel/preset-env', {modules: false}] since some posters said that worked for them, but it didn't work for us. If someone has some better webpack and babel chops is there anything obvious that I could be missing here?