[TalentSignal] properties in schema v4

Hello all, schema.org v4 was released yesterday and has employmentUnit 
<https://schema.org/employmentUnit> and totalJobOpenings 
<https://schema.org/totalJobOpenings> as properties of JobPosting, as we 
proposed a couple of months ago.

Thank you, Phil

-- 

Phil Barker <http://people.pjjk.net/phil>. http://people.pjjk.net/phil
CETIS LLP <https://www.cetis.org.uk>: a cooperative consultancy for 
innovation in education technology.
PJJK Limited <https://www.pjjk.co.uk>: technology to enhance learning; 
information systems for education.

CETIS is a co-operative limited liability partnership, registered in 
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