shelley: use VM's current emoji for favicon

[skybrian]

We modified the server-side favicon generator to poll the integration URL for the emoji that's currently set for the VM. When the user changes the VM's emoji, it will usually change in Shelley on page reload.

Usually because we also added a cache with a one-minute timeout to avoid spamming the integration URL with requests. So, if you change the VM's emoji twice in a row, it won't change immediately the second time. The timeout was arbitrarily chosen and feels a bit long from the user's perspective; maybe it should be shorter?

Other nits: Shelley's favicon is full color and the VM page's emoji is not. I could fix it, but I like color so thought I'd ask first. Also, the way emojis render is inherently font- and platform-dependent.

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