Terminology unified

Changed 'merge' to 'link' in strings to get a more consistent
terminology. The string lables have been left as is.
Affected strings:
* googlelogin-form-merge-title
* googlelogin-form-merge

Also replaced '->' with 'from' to make i18n easier in
* googlelogin-form-unlink

Discussion: https://translatewiki.net/wiki/Thread:Support/About_MediaWiki:Googlelogin-form-merge-title/en

Change-Id: Ia5d9eccf5d1d94fffaf11d261a3eed352f6bb22b