Derive application ID from bot token

[kangalio]

Currently, the bot's application ID must be provided manually if unstable_discord_api features are used.

However, the bot token contains the bot ID, which can be extracted via serenity::utils::parse_token. I think the bot ID is equal to the application ID, so it may be possible to remove the requirement of supplying the application ID manually.

[HarmoGlace]

Unfortunately, it is not true for old bots. But indeed if it is possible, defaulting to the bot id might be a good option

[vicky5124]

Not in all applications the bot id matches the application ID, this is the case on old applications. application_id() should still exist as a manual override, just in case.

[Milo123459]

So this'd essentially work like: When you call .start on the Client, extract the application ID and use that? But if it's an old bot where application_id != client_id you'd have to use a manual override..

[kangalio]

The application ID is now retrieved from gateway which is more robust and just as convenient for the user