Re-add release notes for 1.6.6 and 1.6.7

These release notes were created in 6a03a0bc1f and 05b14af8af, but
somehow didn't make it into the master branch.

This is why http://docs.oscarcommerce.com/en/latest/releases/index.html
currently doesn't mention these releases.

docs/source/releases/index.rst

     v1.6.3
     v1.6.4
     v1.6.5
+    v1.6.6
+    v1.6.7
 
 
 1.5 release branch

docs/source/releases/v1.6.6.rst

+=========================
+Oscar 1.6.6 release notes
+=========================
+
+:release: 2018-01-18
+
+This is Oscar 1.6.6, a bugfix release that adds updated CSS assets that should
+have been included in Oscar 1.6.5.

docs/source/releases/v1.6.7.rst

+=========================
+Oscar 1.6.7 release notes
+=========================
+
+:release: 2018-01-31
+
+This is Oscar 1.6.7, a bugfix release that adds updated dashboard CSS assets that should
+have been included in Oscar 1.6.5.