set release date for 1.8

CHANGELOG.txt

@@ -7,7 +7,7 @@ Changelog
  * View live / draft links in the admin now consistently open in a new window (Marco Fucci)
 
 
-1.8 (xx.xx.xxxx) - IN DEVELOPMENT
+1.8 (15.12.2016)
 ~~~~~~~~~~~~~~~~
 
  * New page privacy options (Shawn Makinson, Tom Miller, Luca Perico, Matt Westcott)

docs/releases/1.8.rst

@@ -1,6 +1,6 @@
-==========================================
-Wagtail 1.8 release notes - IN DEVELOPMENT
-==========================================
+=========================
+Wagtail 1.8 release notes
+=========================
 
 .. contents::
     :local: