Issue #3084069 by Wim Leers, kamalzairig, zrpnr, tinko, webchick, lauriii, tedbow, ckrina: Allow marking themes as experimental

Issue #3023015 by Berdir, kim.pepper, tinko, mikelutz: Replace usages of deprecated constants in file.inc

- blog.module:
   + fixed visual glitch with 'read more' link (blog comments)
   + made it possible to delete your last blog item
   + fixed timestamp bug when previewing a new blog
   + fixed potential problem with blog_save()
   + fixed a few HTML/XHTML tinkos.

- statistics.module:
   + "yoursite.com" should no longer show up under "external referrers"
   + fixed "Referers of the last 31 years 37 weeks 1 hour 46 min 40 sec" bug
   + xhtml-ified

- node.inc:
   + fixed watchdog message

- import.module:
   + small block improvement