Create Admin Endpoint

[cybardev]

Issue

Currently, the admin view is invoked by clicking a button. This is undesirable, as it:

• can be clicked by kids
• can make the game UI cluttered

Proposed solution

Remove the admin view button and have it as a separate webpage.

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Tasks

• make a separate webpage (/admin/index.html)
• validate admin with passphrase (server-side, see Use Server-side Authentication #6)
• connect requests to admin-accessible endpoints (edit server/server.js)