Fixed RestGuildUser not updating inherited RestUser properties

* Fix potential nullref in embedBuilder value setter

* Null check on footer iconUrl

* Adding checks for the other URL properties

* Adding IsNullOrUri extension

* Setting StringExtensions as internal

* Add embed builder extensions

People in #dotnet_discord-net suggested that this should be part of
the lib after I demonstrated it

* Move some extensions into EmbedBuilder [2]

Apparently git didn't like that previous commit

* Fix error with EmbedBuilderExtensions

A summary of issues which happened:
- Git decided to add an amend commit (I told it to quit?)
- VS Code thinks everything is an error so it wasn't helpful
- dotnet decided to think there was no error until I deleted all
  build outputs and rebuild

Sometimes I question my ability to use version control properly.

* Add support for audit log reasons

* Made changes per discussion

* Add various property validation in EmbedBuilder

* Embed URI changes

Changes property types for any URLs in Embeds to System.URI.
Adding field name/value null/empty checks.

* including property names in argumentexceptions

* Adds overall embed length check

Remove IUser.CreateDMChannelAsync / Fix SocketGlobalUser.DMChannel

* remove tip in docs about SocketEntity.Discord

* Expose the 'Fields' collection on EmbedBuilder

After some discussion I decided that there was really no reason to keep this private, and it didn't really go along with the rest of the design of the EmbedBuilder.

This is NOT a breaking change.

Exposing this property should not have any negative effects.

* Don't allow EmbedBuilder's Fields to be set to null

* 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

* Update ChannelHelper.cs

* typo

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