Use "username", "global account" and "local account" consistently

<https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Centralauth-admin-multi-searchprefix/en>

Change-Id: Ib1f0f94ae4bc50dfbb3ec87272faa3f7c87cbea9
Merge "Use "username", "global account" and "local account" consistently"