Make /api/v1/instance and /api/v1/custom_emojis better (#3118)

* Separate commits

From commit dca110ebaa.

* Re-separate commits

From commit 9719387bee.

src/models/mastodon/emoji.ts

@@ -0,0 +1,35 @@
+export type IMastodonEmoji = {
+	shortcode: string,
+	url: string,
+	static_url: string,
+	visible_in_picker: boolean
+};
+
+export async function toMastodonEmojis(emoji: any): Promise<IMastodonEmoji[]> {
+	return [{
+		shortcode: emoji.name,
+		url: emoji.url,
+		static_url: emoji.url, // TODO: Implement ensuring static emoji
+		visible_in_picker: true
+	}, ...(emoji.aliases as string[] || []).map(x => ({
+		shortcode: x,
+		url: emoji.url,
+		static_url: emoji.url,
+		visible_in_picker: true
+	}))];
+}
+
+export function toMisskeyEmojiSync(emoji: IMastodonEmoji) {
+	return {
+		name: emoji.shortcode,
+		url: emoji.url
+	};
+}
+
+export function toMisskeyEmojiWithAliasesSync(emoji: IMastodonEmoji, ...aliases: string[]) {
+	return {
+		name: emoji.shortcode,
+		aliases,
+		url: emoji.url
+	};
+}