Forum search added and some modifications done.

1. Full-text search ising SQLite FTS4 is added.
2. Paging is changed to suit for large pages number.
3. A template for easy adding informational pages without recompilation and with caching is added.
   (Nim Forum searches for HTML-file with given path, if found - shows as the page content,
   if not found - for an RST-file with given path,
   if found - compiles to html, caches to HTML-file and shows as the page content.
   After modifying the RST source-file it's enough to just delete the HTML-file to publish the changes.
   The template is ``textPage``, the example usage - route "/search-help".)
4. The word "Error" was shown in Forum's title (``title`` tag) in preview-mode (without any error).
   Changed to show "Nimrod Forum - Preview" as title for preview mode.