Merge pull request #89 in ZP/z-push from feature/ZP-735-add-option-to-delete-hierarchy-cache to develop

* commit 'e12c6cc5':
  ZP-735 Clearify the device switch and that this is the folder hierarchy.
  ZP-735 Add option to resync only the hierarchy of a specific user, device or all. When forcing resync, delete first all states of synchronized folders and afterwards the hierarchy, so the necessary information is still available. Reset contentData & ignoredMessages of the device ONLY when removing the hierarchy synckey, not when setting a new one.