Claned up mentions, added sanitize handler to user/role/channel mentions

src/Discord.Net.Core/Entities/Messages/Mentions/ChannelMentionHandling.cs

@@ -4,6 +4,7 @@
     {
         Ignore = 0,
         Remove,
-        Name
+        Name,
+        Sanitize
     }
 }

src/Discord.Net.Core/Entities/Messages/Mentions/RoleMentionHandling.cs

@@ -4,6 +4,7 @@
     {
         Ignore = 0,
         Remove,
-        Name
+        Name,
+        Sanitize
     }
 }

src/Discord.Net.Core/Entities/Messages/Mentions/UserMentionHandling.cs

@@ -5,6 +5,7 @@
         Ignore = 0,
         Remove,
         Name,
-        NameAndDiscriminator
+        NameAndDiscriminator,
+        Sanitize
     }
 }