API: Deprecate FormatEmojiAPI in favor of a simpler function

[mavolin]

FormatEmojiAPI takes two parameters, the first one being optional, which is not very idiomatic.
Also FormatEmojiAPI doesn't even return a EmojiAPI type, but a string.
Also, if we're doing this APIEmoji might be a better name, since this is an emoji used by the API and not an API for emojis.
Therefore, I propose to either deprecate or delete FormatEmojiAPI in favor of two new functions:

func NewAPIEmojiFromGuildEmoji(id discord.Snowflake, name string) APIEmoji {
    return name+":"+id.String()
}

func NewAPIEmojiFromUnicode(emoji string) APIEmoji {
    return name
}

Additionally, APIEmoji should get a URLEncode() string method, that URL encodes the emoji for safe API usage.

Can hand in a PR for this

[diamondburned]

You're only supposed to use NewAPIEmoji for custom emojis anyway, there's no point in using it for normal emojis. This could be documented instead of having a dummy function return the same string.

[mavolin]

But it's used by React which allows reactions with custom and with built in/unicode emojis.

[diamondburned]

Well, yes, but APIEmoji is just a type alias to string; it's only meant to add extra connotation on the differences of syntax for custom emojis between contexts (messages, in code, in API).

[mavolin]

K, I'll replace it with a comment