Reverting center channel margins and fixing the channel names (#2823)

Fixing margin for channel intro

Adjusting margins for preformatted text

webapp/sass/layout/_markdown.scss

@@ -29,8 +29,8 @@
     pre {
         border: 1px solid rgba(221,221,221,0.2);
         border-radius: .25em;
-        margin: 0;
-        padding: 0px;
+        margin: 5px 0;
+        padding: 0;
         text-align: left;
     }