Read query parameters (key, sigAlgo and source) from URL

[muttoni]

Closes #38

Description

The Create Account flow now automatically prefills publicKey and sigAlgo with the following query URL parameters:

• key : the public key (as per issue Query parameters for passing values #38)
• sig-algo: the signature algorithm (as per issue Query parameters for passing values #38), defaulting to ECDSA_P256

It also supports an extra source parameter, which can be used to identify the traffic source (e.g. kittyitems). This information is included as an additional property in the "Faucet: Create Account" mixplanel event.

DISCLAIMER: I am not very familiar with the Typescript/React/Next.js stack, so apologies if this is not idiomatic or is not adhering to Next.js best practices. After exploring a couple options, I used getInitialProps to read the query params and provide context to the page as props. I previously tried a simple client-side solution using next/router directly in the form component but it was not prefilling correctly since it was being pre-rendered on the server without re-hydrating.

DISCLAIMER #2: I was not able to get docker running, so I limited my testing to the frontend -- it appears to work fine (parameters can be empty, a nonexistent sigAlgo defaults to ECDSA, etc), but can someone can please E2E test this before merging just in case? (e.g. check creation success, mixpanel events, etc)

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• Targeted PR against main branch
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work
• Code follows the standards mentioned here
• Updated relevant documentation
• Re-reviewed Files changed in the Github PR explorer
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