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)

* Add IChannel.Nsfw, RequireNsfw precondition

* Refactored IChannel.Nsfw to IsNsfw, removed NsfwUtils

Per pull-request feedback

* proper nsfw channel check

* Update ChannelHelper.cs

* typo

Resolves #469

this was a big one

I'm not entirely sure this is the proper way to fix this. However, it seems like
SendFileAsync returns the Task<RestUserMessage>, which would exit the using
block, disposing of the FileStream potentially before the Task is finished.

Changed to await the result of the task before exiting the using block.