Merge remote upstream

+ Starting from this commit, all return types for tasks will use style similar to most documentations featured on docs.microsoft.com

References:
https://docs.microsoft.com/en-us/dotnet/api/microsoft.entityframeworkcore.dbcontext.-ctor?view=efcore-2.1
https://docs.microsoft.com/en-us/dotnet/api/system.io.filestream.readasync?view=netcore-2.1
https://docs.microsoft.com/en-us/dotnet/api/system.io.textwriter.writelineasync?view=netcore-2.1#System_IO_TextWriter_WriteLineAsync_System_Char___
And many more other asynchronous method documentations featured in the latest BCL.

This dispatch is undocumented and only used by user accounts.

* Mark guild as optional for invite

* Mark partial InviteMetadata members as Optional<T>

* Some of them aren't sent when requesting through the general GET invite endpoint

* Remove GetInviteParams

* It was kinda stupid in the first place, might as well always get the count instead of having to ask the user whether they want the two fields filled or not.

* Add ChannelType property

* Add vanity invite support

* References
https://docs.microsoft.com/en-us/dotnet/api/system.argumentnullexception?view=netframework-4.7.2
https://docs.microsoft.com/en-us/dotnet/api/system.platformnotsupportedexception?view=netframework-4.7.2
https://docs.microsoft.com/en-us/dotnet/api/system.badimageformatexception?view=netframework-4.7.2

* Fix ' />' caused by Agent Smith oddities
* Fix styling to be more specific about the mention of IDs

* Solved 4003 Authentication Error (Race Condition)

* Set static interval & removed hello case

+ Some contributions from @purpledank

* InvalidPathChars does not exist in NS1.3; replaced with GetInvalidPathChars instead.

* ConnectAsync(bool external) API implemented

This allows developers to handle the AudioClient externally (for example with Lavalink)

* Modify ConnectAsync API and add DisconnectAsync to IAudioChannel

* Update summary message on IAudioChannel.DisconnectAsync()

* Review changes:
- Fix `if (!external)` styling
- Remove unecessary ConnectAsync overload

* SocketVoiceChannel overload update

* Update IAudioChannel.ConnectAsync()
- Remove ConfigAction from parameters
- Add SelfDeafen/SelfMute to parameters

* Remove SocketVoiceChannel.ConnectAsync() default params (Inherit)