Add REST endpoint to create empty change

This change is based on change-fabricator plugin [1] and is a
prerequisite for inline edit feature to support 100% Gerrit workflow
in browser.

[1] https://github.com/davido/gerrit-change-fabricator

Bug: Issue 2338
Change-Id: Idf290c14d5a478479fe1cb916ee227aa11435fcd
7 files changed