Override some Unicode emoji for display

[gnprice]

We should probably port from zulip-mobile its "override" logic that applies to a handful of Unicode emoji:
https://github.com/zulip/zulip-mobile/blob/c979530d6804db33310ed7d14a4ac62017432944/src/emoji/codePointMap.js#L3-L33
https://github.com/zulip/zulip-mobile/blob/c979530d6804db33310ed7d14a4ac62017432944/src/emoji/data.js#L116

See comments there for motivation.

The first step will be to check that the un-overridden emoji actually do look wrong in zulip-flutter, as they did in zulip-mobile. A PR for this should include before/after screenshots that demonstrate the change.