chore: simplify storing and sending custom emoji's

- 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.

modules/markup/html.go

@@ -1142,7 +1142,7 @@ func emojiShortCodeProcessor(ctx *RenderContext, node *html.Node) {
 		converted := emoji.FromAlias(alias)
 		if converted == nil {
 			// check if this is a custom reaction
-			if _, exist := setting.UI.CustomEmojisMap[alias]; exist {
+			if setting.UI.CustomEmojisLookup.Contains(alias) {
 				replaceContent(node, m[0], m[1], createCustomEmoji(alias))
 				node = node.NextSibling.NextSibling
 				start = 0