UserTypeReader ArgumentNullException in DM

Automatic animated avatar detection

Fixed GetReactionsUsersAsync returning an empty enumerable

Update audio_ffmpeg.cs

Fixed the example precondition attribute

Add installing with nuget using VSC to guides

AudioApplication is a required argument

Thanks @MinnDevelopment for his awesome work on
the JDA guide that had no influence here at all.

🥄 better docs soon come

ConnectAsync -> StartAsync

This should resolve a plethora of relates issues, including user downloading not working (#542), possibly #531

A null conditional operator was required at line 70 to avoid a nullref.

Conflicts:
	src/Discord.Net.WebSocket/DiscordSocketClient.cs

Check that messages older than two weeks are not passed to bulk delete

Not pretty, but it works.

Connection/API lag could cause messages right on the edge of 14 weeks to fail on Discord's end when they pass our checks.

Resolves #469