Changed Menu to only require UserId and ChannelId

[andersgaustad]

Makes it possible to create a new menu exclusively from UserId and ChannelId without requiring a message.
Especially useful for creating a Menu object as a response to a slash command (as no Message object exists in this context, but the ids do).

For example:

pub async fn run(ctx: &Context, command_interaction: &ApplicationCommandInteraction) {

     ... code here ...

    let pages = create_pages();
    let menu = Menu::new_from_ids(ctx, &command_interaction.user.id, &command_interaction.channel_id, &pages, MenuOptions::default());

     ... more code here ...

}