Sync server-specific unicode emoji with local cache at startup

[neiljp]

Our current set of supported unicode emoji are irregularly fetched from the current master/main branch using tools/fetch-unicode-emoji-data, translated using another script, and then integrated via a commit with the resulting changes. This uses a file internal to the server source exposed via github.

Zulip 6.0 is expected to support a way to explicitly expose the unicode emoji in use and support providing a url to fetch from and a way to locally cache the url contents via a hash.

The current planned approach is to use cache-control headers, which should be achievable using something like requests-cache. This may also connect with work towards a central configuration location, eg. #678

Unicode emoji are not updated frequently, but as part of this work we likely want to ensure that code that handles emoji and reactions can cope if the emoji are updated, such as if a server is upgraded. We can check with the web app to see how they handle some of these situations, but an error like zulip/zulip-mobile#5430 for mobile motivated this addition to the server.

We'll also want to consider interoperating with older server versions that don't provide this.

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• Discussion at #api design > currently supported emoji.
• Server PR zulip/zulip#22476
• Mobile PR zulip/zulip-mobile#5460

[zulipbot]

Hello @zulip/server-emoji members, this issue was labeled with the "area: emoji" label, so you may want to check it out!