Combine notes from RCs into X.Y.Z.rst

[berkerpeksag]

Currently, 2.7.15.rst only shows changes between 2.7.15rc1 and 2.7.15 final, not between 2.7.14 final and 2.7.15 final. I think this creates a bit confusion and breaks the old workflow. We can't now just link to https://github.com/python/cpython/blob/2.7/Misc/NEWS.d/2.7.15.rst at https://www.python.org/downloads/release/python-2715/ since it doesn't combine changes from https://github.com/python/cpython/blob/2.7/Misc/NEWS.d/2.7.15rc1.rst and https://github.com/python/cpython/blob/2.7/Misc/NEWS.d/2.7.15.rst.

See python/pythondotorg#1252 for more details.

[larryhastings]

Please see python/pythondotorg#1252 (comment) for my comments.