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/IMessageChannelExtensions.cs

@@ -30,7 +30,7 @@ public static class MessageChannelExtensions
         {
             SmartEmbedText set => channel.SendAsync(set.PlainText, set.GetEmbed().Build(), sanitizeAll: sanitizeAll),
             SmartPlainText st => channel.SendAsync(st.Text, null, sanitizeAll: sanitizeAll),
-            SmartEmbedTextArray arr => channel.SendAsync(arr.PlainText,
+            SmartEmbedTextArray arr => channel.SendAsync(arr.Content,
                 embeds: arr.GetEmbedBuilders().Map(e => e.Build())),
             _ => throw new ArgumentOutOfRangeException(nameof(text))
         };