Feature/single account multiple athletes

[ikusteu]

Fixes #904

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[ikusteu]

Looks great in general. Needs a fix though.

I tested this in production (AFAIU there are no changes to cloud functions in this PR, so it's safe to test it through the preview channel deployed by the CI) and I was able to:

• register a new athlete
• see the newly registered athlete amongst the choices in /select_account

I was not able to:

• see my other, already existing, account with the same email
• see my other, already existing, account with the same phone number

I've used eisbuk.web.app (the preview channel) to test this out: I've created a new athlete (with my existing account). I was able to see both and navigate between...

Did you (maybe) mean: You already had two accounts with the same email and expected to see them both in the selection (now with the multiple athlete update)...if so, yeah, that should be working, but if it's not, maybe we should debug that together...

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[github-actions[bot]]

This pull request has conflicting changes, the author must resolve the conflicts before this pull request can be merged.

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