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.

templates/base/head_script.tmpl

@@ -13,7 +13,7 @@ If you introduce mistakes in it, Gitea JavaScript code wouldn't run correctly.
 		assetVersionEncoded: encodeURIComponent('{{AssetVersion}}'), // will be used in URL construction directly
 		assetUrlPrefix: '{{AssetUrlPrefix}}',
 		runModeIsProd: {{.RunModeIsProd}},
-		customEmojis: {{CustomEmojis}},
+		customEmojis: new Set({{CustomEmojis}}),
 		csrfToken: '{{.CsrfToken}}',
 		pageData: {{.PageData}},
 		notificationSettings: {{NotificationSettings}}, {{/*a map provided by NewFuncMap in helper.go*/}}