Implement react i18next

[pettinarip]

This PR implements react-i18next and creates & loads the locales in a different way than #8866 to avoid having memory consumption problems.

Description

Preview deploy: https://ethereumorgwebsitei18nrework.gatsbyjs.io/en/

In short, instead of mergeTranslations in one big file, all the json files under src/intl are loaded as they are, keeping the same namespaces. Then, on each page, we query only the necessary namespace that that page needs.

To do this efficiently, we had to reorder some translations from one namespace to another. Basically to avoid loading an entire namespace just for 1 translation. For example, adding new strings on common.json is very expensive since that file is loaded on all the pages we have (at the moment 12k pages), so I tried my best to reduce that file to the minimum.

Other things done in this PR:

• createLocales script was completely redone. Check docs/locale-process.md for more information
• mergeTranslations script is not more needed and it was deleted
• new utility unusedTranslations script added
• new utility reorgIntlFiles script added
• yarn.lock was regenerated with latest packages versions (this one mitigates a wall of warnings we had on dev env)
• refactored page queries to fetch locales using the correct namespaces

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Note: for ease to review purposes, I didn't run the reorgIntl script yet. I'll include that on a different PR. What that script is going to do is to apply the same reorgs I did on src/intl/en to all the other languages.

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Note2: stress tests were executed here #9455

[pettinarip]

Now the namespaces have the same name as the files under src/intl.

[pettinarip]

This slug is incorrrect. This slug is always the /en/ version. To fix this, now we are getting it from the pageContext.

[corwintines]

Nice one! This really improves load times from some testing. Noticed a couple things. Happy to sync up and talk about this.

[corwintines]

LGTM, approving since dev will be merged into this branch that will include #9537

[konopkja]

@pettinarip @corwintines reviewed light / dark themes, mobile layouts and 2 different languages. Mostly looks goods, only few things down here:

1. find wallet page select box shows code string
   

2. stablecoins page doesnt show button as prod version
   
   vs prod version

3.tutorials "clear" button shows as string instead

[pettinarip]

Thanks @konopkja for the review 💪🏼

1. Good catch.
2. This branch is pretty old. Soon I'll update it and we should see the same buttons as in prod 👍🏼
3. Good catch.

tldr, I'll fix those missing translations & update the branch.