Fix multiline quote rendering in HTML

Fixes #756

DiscordChatExporter.Core/Exporting/Writers/Html/PreambleTemplate.cshtml

@@ -113,8 +113,7 @@
             margin: 0.05em 0;
         }
 
-        .quote::before {
+        .quote-border {
-            content: "";
             margin-right: 0.5em;
             border: 2px solid @Themed("#4f545c", "#c7ccd1");
             border-radius: 3px;

DiscordChatExporter.Core/Exporting/Writers/MarkdownVisitors/HtmlMarkdownVisitor.cs

@@ -39,7 +39,8 @@ internal partial class HtmlMarkdownVisitor : MarkdownVisitor
             FormattingKind.Strikethrough => ("<s>", "</s>"),
             FormattingKind.Spoiler => (
                 "<span class=\"spoiler-text spoiler-text--hidden\" onclick=\"showSpoiler(event, this)\">", "</span>"),
-            FormattingKind.Quote => ("<div class=\"quote\">", "</div>"),
+            FormattingKind.Quote => (
+                "<div class=\"quote\"><div class=\"quote-border\"></div><div class=\"quote-content\">", "</div></div>"),
             _ => throw new ArgumentOutOfRangeException(nameof(formatting.Kind))
         };