Prevent users from choosing the name of an already existing page

This should prevent accidents like BLOG-76, or if a non-blog page already exists, then creating a blog post with the same name will add a BlogPost object to that page, making the actual page content invisible to viewers.

Also if a blog post with the same name already exists, people might change the existing blog post (maybe wondering why there is already nearly the right content prefilled) instead of creating a new one without noticing. (This is the problem case I got reported most often).

The idea is to prevent name clashes by adding a _NN suffix to the user given title in the blog post name, so the users will not have to worry about picking a new name manually.