Change "client hub" to "CMS" everywhere.

Pivotal Tracker story: https://www.pivotaltracker.com/story/show/64271218

Shorthand: CMS
Longhand: Content Management System
Name: g5-content-management-system

Related PRs
https://github.com/g5search/g5-client-hub/pull/264
https://github.com/g5search/g5-client-app-creator/pull/23
https://github.com/g5search/g5-sibling-deployer/pull/10