New async

1. Full-text search using 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.
5. When editing existing post, after submiting the user was redirected to the last post in the thread.
   Now redirection is to the same post that was edited.