Fix everyone mention. (#755)

* Update RestRole.cs

Fix everyone mention.

* Update SocketRole.cs

Fix everyone mention.

* I'm good at this, I swear.

src/Discord.Net.Rest/Entities/Roles/RestRole.cs

@@ -19,7 +19,7 @@ namespace Discord.Rest
 
         public DateTimeOffset CreatedAt => SnowflakeUtils.FromSnowflake(Id);
         public bool IsEveryone => Id == Guild.Id;
-        public string Mention => MentionUtils.MentionRole(Id);
+        public string Mention => IsEveryone ? "@everyone" : MentionUtils.MentionRole(Id);
 
         internal RestRole(BaseDiscordClient discord, IGuild guild, ulong id)
             : base(discord, id)

src/Discord.Net.WebSocket/Entities/Roles/SocketRole.cs

@@ -23,7 +23,7 @@ namespace Discord.WebSocket
 
         public DateTimeOffset CreatedAt => SnowflakeUtils.FromSnowflake(Id);
         public bool IsEveryone => Id == Guild.Id;
-        public string Mention => MentionUtils.MentionRole(Id);
+        public string Mention => IsEveryone ? "@everyone" : MentionUtils.MentionRole(Id);
         public IEnumerable<SocketGuildUser> Members 
             => Guild.Users.Where(x => x.Roles.Any(r => r.Id == Id));