Solve a challenge before requesting guest account

Daniel Gerhardtrequested to merge
altcha-challenge into master

A PoW-based challange is solved using Altcha. The challege is retrieved from the backend and the solution is sent back in exchange for a JWT. This can be used for authentication for endpoints which don't require a user account. For now, it is used for creating guest accounts.

Related: arsnova-server#344 (closed)

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