- Using a map to store the CustomEmojis is unncessary, the values can be
derived by the key easily. We still need to know if a certain name is
custom emoji, so store the custom emojis in a `Set`.
- Instead of constructing a map in `window.config`, construct a `Set`
and reconstruct the value in `emoji.js`. This has the main benefit of
reducing the amount of text being sent for each request, which is quite
noticable if a Forgejo instance has many (>1000) custom emojis.
- Remove the default value of `CustomEmojisMap`, it will be constructed anyway.
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