Partly respecting the kiwix-serve --nosearchbar option

`--nosearchbar` option of `kiwix-serve` (despite its misleading name)
was used to disable the entire taskbar. This commit accounts for the
existence of that option only partially:

1. Links to books on the welcome/library page are affected - by default
   books are displayed in the viewer, but in a kiwix-serve instance run
   with --nosearchbar books are loaded in the top window.

2. The `/viewer` endpoint is enabled unconditionally, so if anyone
   enters the viewer URL in the address bar they will see books in the
   viewer.

test/server.cpp

@@ -184,7 +184,7 @@ R"EXPECTEDRESULT(      href="/ROOT/skin/index.css?cacheid=3b470cee"
           src: url("/ROOT/skin/fonts/Roboto.ttf?cacheid=84d10248") format("truetype");
     <script src="/ROOT/skin/isotope.pkgd.min.js?cacheid=2e48d392" defer></script>
     <script src="/ROOT/skin/iso6391To3.js?cacheid=ecde2bb3"></script>
-    <script type="text/javascript" src="/ROOT/skin/index.js?cacheid=2f5a81ac" defer></script>
+    <script type="text/javascript" src="/ROOT/skin/index.js?cacheid=12321f62" defer></script>
 )EXPECTEDRESULT"
     },
     {