Add banner (large) image

PluralKit.Bot/Commands/SystemEdit.cs

@@ -218,7 +218,60 @@ namespace PluralKit.Bot
             else
                 await ShowIcon();
         }
-        
+
+        public async Task BannerImage(Context ctx)
+        {
+            ctx.CheckSystem();
+
+            async Task ClearImage()
+            {
+                await _db.Execute(c => _repo.UpdateSystem(c, ctx.System.Id, new SystemPatch {BannerImage = null}));
+                await ctx.Reply($"{Emojis.Success} System banner image cleared.");
+            }
+
+            async Task SetImage(ParsedImage img)
+            {
+                await AvatarUtils.VerifyAvatarOrThrow(img.Url, isFullSizeImage: true);
+
+                await _db.Execute(c => _repo.UpdateSystem(c, ctx.System.Id, new SystemPatch {BannerImage = img.Url}));
+
+                var msg = img.Source switch
+                {
+                    AvatarSource.Url => $"{Emojis.Success} System banner image changed to the image at the given URL.",
+                    AvatarSource.Attachment => $"{Emojis.Success} System banner image changed to attached image.\n{Emojis.Warn} If you delete the message containing the attachment, the banner image will stop working.",
+                    AvatarSource.User => throw new PKError("Cannot set a banner image to an user's avatar."),
+                    _ => throw new ArgumentOutOfRangeException()
+                };
+
+                // The attachment's already right there, no need to preview it.
+                var hasEmbed = img.Source != AvatarSource.Attachment;
+                await (hasEmbed 
+                    ? ctx.Reply(msg, embed: new EmbedBuilder().Image(new(img.Url)).Build()) 
+                    : ctx.Reply(msg));
+            }
+
+            async Task ShowImage()
+            {
+                if ((ctx.System.BannerImage?.Trim() ?? "").Length > 0)
+                {
+                    var eb = new EmbedBuilder()
+                        .Title("System banner image")
+                        .Image(new(ctx.System.BannerImage))
+                        .Description("To clear, use `pk;system banner clear`.");
+                    await ctx.Reply(embed: eb.Build());
+                }
+                else
+                    throw new PKSyntaxError("This system does not have a banner image set. Set one by attaching an image to this command, or by passing an image URL or @mention.");
+            }
+
+            if (await ctx.MatchClear("your system's banner image"))
+                await ClearImage();
+            else if (await ctx.MatchImage() is {} img)
+                await SetImage(img);
+            else
+                await ShowImage();
+        }
+
         public async Task Delete(Context ctx) {
             ctx.CheckSystem();