Create `index` flag in Page Model, only allow one page to be index.

* The `index` flag indicates that the page is the "Index" of the site
  and should be displayed when the user loads `/`.
* Only one page can be the index. When a page is set as the index, we
  automatically unset all "index" pages before we save.
* Created `Page.index` which returns the Index page.
* If there are no other pages, or the current page is the only page
  in the database, then force this page as the index.