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/_post.scss

@@ -167,7 +167,7 @@
     text-align: center;
     top: 0;
     width: 100%;
-    z-index: 6;
+    z-index: 11;
 
     .overlay__indent {
         @include clearfix;
@@ -361,7 +361,7 @@
 .post-create__container {
     @include flex(0 0 auto);
     width: 100%;
-    z-index: 1000;
+    z-index: 9;
 
     label {
         font-weight: normal;