Support proposed/* branches in release scripts

mpcut.sh and mpdelivery.sh scripts were used to cut milestone-proposed
branches for milestones and pre-release, and then tag and upload
development milestones, RCs and final releases.

This change replaces them by a rccut.sh script which will cut a
proposed/foo branch for pre-release (at RC1 time), and a rcdelivery.sh
script which will tag and upload RCs and final releases.

These new scripts also take simpler input than the old versions, by
taking advantage of ms2version.py to calculate version numbers and
validate milestone pages (although in the swift case, we still rely on
a command-line argument to convert series to version number).

Change-Id: Ied44469881caa1f9d1fd2035d2f2f8bfae7f2885
Merge "Support proposed/* branches in release scripts"