Multiple Ui fixes (#6272)

* PLT-6453 - Improving headings in the web app

* PLT-6414 - Fixing view members modal issue

* PLT-5863 - Fixing alignment in new channel menu

* PLT-6391 - Fixing code block overlay

webapp/sass/layout/_markdown.scss

@@ -1,11 +1,36 @@
 @charset 'UTF-8';
 
+h1 {
+    font-size: 28px;
+}
+
+h2 {
+    font-size: 25px;
+}
+
+h3 {
+    font-size: 22px;
+}
+
+h4 {
+    font-size: 19px;
+}
+
+h5 {
+    font-size: 15px;
+}
+
+h6 {
+    font-size: 1em;
+}
+
 .markdown__link {
 }
 
 .markdown__heading {
     font-weight: 700;
     line-height: 1.5;
+    margin: 10px 0;
 }
 
 .markdown__paragraph-inline {