page_manager 8.x-1.0-alpha8

Changes since 8.x-1.0-alpha7:

• Consolidate PageAwareVariantInterface check to DisplayVariantFormBase.
• Remove redundant PageFormBase::save().
• Remove obsolete third parameter passed to resolveConditions().
• Do not interfere with variant selection conditions, allow block pages with no blocks to be used.
• Altering display variant forms is too hard.
• PageInterface and schema should extend config entity.
• Adjust for the deprecation of Utility\String in #2454447.
• #2419081 by Berdir: Container is being serialized if you have a block form with #ajax