navbar: Add globe icon to navbar for web public streams.

Prior to this commit, the navbar didn't display web public streams as
any different from ordinary streams.

Now, we show a globe icon for web public streams. This commit also
adds a node test for the same.

We also modified the navbar behaviour table, which is the following
dropbox paper:
https://paper.dropbox.com/doc/Navbar-behavior-table--A0sp57z~7R5PeHuxHL__Gn5ZAg-cNOGtu7kSdtnKBizKXJge

static/js/filter.js

@@ -529,6 +529,7 @@ Filter.prototype = {
         return {
             _stream_name: sub.name,
             _is_stream_private: sub.invite_only,
+            _is_web_public: sub.is_web_public,
         };
     },
 
@@ -543,6 +544,9 @@ Filter.prototype = {
             if (this._stream_params._is_stream_private) {
                 return 'lock';
             }
+            if (this._stream_params._is_web_public) {
+                return 'globe';
+            }
             return 'hashtag';
         case 'is-private':
             return 'envelope';