Fix for issue #1555 and #1620: Page path rewriting when published page becomes the HOME page

* when CMS_MODERATOR = True:
  if the draft page is unpublished, the public version is also unpublished
* when determining the home page, ignore publication_date and publication_end_date
  this ensures that the home page path rewriting logic happens even if the page will only become available
     somewhere in the future
* change the post_save_page signal handler so that it also updates the:
  ** paths(titles) of the currently saved page
  ** if the currently saved page becomes the new home page, then update the paths(titles) of the previous home page
  ** if the currently saved page used to be the home page, then update the paths(titles) of the new home page
* add pre/post delete signal handlers that: if the currently deleted page was the home page
  then update the path of the new home page
* Added unit tests for all the above functionality

This ensures that:
* we will always have AT MOST one page with the '' (empty) path
* no matter what publishing method is used (tick the publish/unpublish checkbox in the page hierarchy,
    tick the 'is published' checkbox from the 'change page' view OR click 'publish' while doing frontend editing)
    the generated paths are consistent
Removed unused import
Better naming to the delete page signal handlers.
Renamed pre/post delete signal handlers and reduced the number of queries.
Merge pull request #2 from pbs/issue_1555

    Fix for issue #1555 and #1620: Page path rewriting when published page becomes the HOME page