feat: login by passing API token in URL #hash

[yusefnapora]

This adds the ability to set the URL hash to #authToken=<your-api-key> and use it to authenticate to the API.

It adds a /user/meta route to return the issuer and publicAddress, which we were previously getting from getMagic().user.getMetadata().

The "is logged in" check in _app.js was previously using getMetadata() to set the user object that gets passed through to UserContext. Since we can no longer assume that all logged in users will have a valid magic link session, I had to add a server route to return similar data (issuer and publicAddress).

Stores the token from the fragment in localStorage, so we can clear the fragment and keep accessing the token. Maybe sessionStorage is more appropriate? I forget when to use which, lol.

This probably needs some cleanup, and it's branched off of #1864, so it's a draft for now.

@redaphid will you play with this next week? I probably won't have time to work on it, but I can answer any questions you might have.

To use it with the test user in the local development env, follow this link while the dev server is running from this branch (assuming it's on the default port).

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