mwext: Add Disambiguator (mwext-check-jobs template)

Pushed the default templates for now:

 - '{name}-{ext-name}-merge'
 - '{name}-{ext-name}-lint'
 - '{name}-{ext-name}-whitespaces'
 - '{name}-{ext-name}-jslint'
 - '{name}-{ext-name}-install'
 - '{name}-{ext-name}-testextensions-{mwbranch}'
 - '{name}-{ext-name}-phpcs-HEAD'

Change-Id: I6046c650b4c0a8bd94e0709c2b96e187b7185cd7