Remove dependencies on imports from Classic UI packages

[davisagli]

Related to plone/Products.CMFPlone#3953

• plone.app.layout
  • This is the main package that will hold the classic UI views and templates. plone.restapi needs to stop depending on it.
  • Move plone.app.layout.navigation.navtree.buildFolderTree somewhere
  • Move history fetching logic from ContentHistoryViewlet somewhere
• plone.app.workflow
  • This package is mostly for the Classic UI sharing view which will move to plone.app.layout
  • Move LocalrolesModifiedEvent to plone.base
  • Move ISharingPageRole to plone.base
  • Move merge_search_results to plone.app.users
• plone.app.content
  • This package has various Classic UI views that will move to plone.app.layout. After that, it is a pretty small package which maybe doesn't need to exist (but removing it can be low priority)
  • Move INameFromTitle to plone.base
  • Move vocabulary permissions mapping to plone.app.vocabularies

(Of course, we need to keep backwards-compatibility imports for Plone 5.)