Full support for embed arrays in .greet/.bye, .say and other commands which use embeds

- Website to create them is live at eb2.nadeko.bot (it will soon be replacing eb.nadeko.bot)
- Embed arrays don't have a plainText property (it's renamed to 'content')
- Embed arrays use color hex values instead of an integer
- Old embed format will still work
- There shouldn't be any breaking changes

src/NadekoBot/_Extensions/Extensions.cs

@@ -26,7 +26,7 @@ public static class Extensions
             SmartEmbedTextArray set => msg.ModifyAsync(x =>
             {
                 x.Embeds = set.GetEmbedBuilders().Map(eb => eb.Build());
-                x.Content = set.PlainText?.SanitizeMentions() ?? "";
+                x.Content = set.Content?.SanitizeMentions() ?? "";
             }),
             SmartPlainText spt => msg.ModifyAsync(x =>
             {