* Partial fix of #810, addresses critical connection issues.

* Implement fix for REST.

* Implement fix on RestChannel.

* Add GetReactionUsersAsync(IEmote) overload

Resolves #730

* Remove obsolete GetReactionUsersAsync(string) overload

* Changed Guild#DefaultChannel to resolve the first accessible channel

Resolves #776

This change is inline with hammerandchisel/discord-api-docs#329

RestGuild#DefaultChannelId is now obsolete and will throw a
NotSupportedException.

* RestGuild#DefaultChannelId will fall back to the guild ID

Adding an exception here would be a breaking change, so this was agreed
to fall back to the previous behavior, which would just return the guild
ID.

IChannel.IsNsfw will now return false when being used on any channel
that is not an ITextChannel. When being used on an ITextChannel, this
will now account for the API flag, and fall back to the channel name.

(this is gross design, thanks discord)

* Update RestRole.cs

Fix everyone mention.

* Update SocketRole.cs

Fix everyone mention.

* I'm good at this, I swear.

* Make ShardedCommandContext derive from SocketCommandContext

* Explicitly re-implement ICommandContext.Client

* Add support for audit log reasons

* Made changes per discussion

* Add SocketRole.Members property

* Change Members to IEnumerable.

This resolves #702

Remove IUser.CreateDMChannelAsync / Fix SocketGlobalUser.DMChannel

* Made GetOrCreateUser always call AddRef and added check to PRESENCE_UPDATE to avoid readding users who have been removed from guilds

* Removed AddRef as per dev guild discussion

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

* Replace Where.FirstOrDefault with FirstOrDefault

* Replace Where+FirstOrDefault in ClientHelper

The previous commit prevents any connections, since the initial presence update is sent while the client is still in the 'connecting' state, rather than the 'connected' state.

This resolves the original issue by preventing a nullref, and the more recent issue by only throwing a detailed exception when the CurrentUser is null (the client isn't logged in).

This prevents a later, less detailed nullref, when attempting to set the CurrentUser's presence data.

This also removes a redundant CurrentUser assignment in the SetGameAsync method, since this will be set later on in the SendStatusAsync method.