New 'OutputPageAfterGetHeadLinksArray' hook, allowing extensions to modify the return...

New 'OutputPageAfterGetHeadLinksArray' hook, allowing extensions to modify the return value of OutputPage#getHeadLinksArray

Code from wikiHow codebase, where this hook is used by the following extensions:
* AlternateDomain -- used to remove certain links altogether and change the contents of other elements (e.g. <meta description="..." />)
* hooks (PageHooks) -- used to hide certain links for anons on noindexed pages to avoid leaking article info to Googlebot
* QADomain -- used to remove certain elements and correct <meta keywords="..." /> tags not to mention "wikiHow" if that string is present
* search (LSearch) -- used to remove canonical URL on Special:LSearch for SEO

Change-Id: I4a9ceb343bb5c0b4eb79e4589d36c3790938f8a9