Add support for properties hst:homepagerefid and hst:pagenotfoundrefid for configuring the mounts

For each subsite, we have to define a hst:homepage and a hst:pagenotfound matching a sitemap item. It would be nice to have properties like hst:homepagerefid and hst:pagenotfoundrefid, matching a sitemap item by the refId instead of the node name, which differs according to the language.

Example for two subsites (ENglish and FRench) :
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EN/sitemap

• home { refId=homePage }
  - page-not-found { refId = pageNotFound)
  
  FR/sitemap
  - accueil { refId=homePage }
• page-non-trouvée { refId=pageNotFound }

BEFORE

hosts

• EN { homepage = home, pagenotfound = page-not-found, locale = en_UK }
• FR { homepage = accueil, pagenotfound = page-non-trouvée, locale = fr_FR }

AFTER

hosts

{ homepagerefid = homePage, pagenotfoundrefid = pageNotFound }

• EN { locale = en_UK }
• FR { locale = fr_FR }

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Thus, it would save a lot of time, and be less error-prone for projects with a lot of subsites...