Add "back to home" button at top of help pages.

This adds a button that will bring a user back to the home
of the help section "/help".

static/styles/portico.css

@@ -1159,6 +1159,21 @@ a.bottom-signup-button {
     font-weight: 600;
 }
 
+.markdown .back-to-home {
+    float: right;
+    margin-top: 13px;
+}
+
+.markdown .back-to-home .icon-vector-chevron-left {
+    position: relative;
+    top: 1px;
+    margin-right: 5px;
+}
+
+.markdown .back-to-home a:hover {
+    text-decoration: none;
+}
+
 .markdown strong {
     font-weight: 600;
     white-space: pre;

templates/zerver/help/main.html

@@ -6,6 +6,9 @@
 
 <div class="app terms-page">
     <div class="app-main markdown">
+        <div class="back-to-home">
+            <a href="/help/"><div class="icon-vector-chevron-left"></div>Home</a>
+        </div>
     {{ article|render_markdown_path }}
         <div id="footer" class="documentation-footer">
         <hr />