FIX: i18n support in LookupField

Fixed i18n support in LookupField when value is empty (read-only complement of DropdownField)
Merge pull request #3534 from jelicanin/3.1

FIX: i18n support in LookupField
FIX: Deliberately clear partial cache blocks on flush (fixes #1383)

Move property to top of class definition

Move property to top of class definition
Merge pull request #3530 from kinglozzer/pulls/flush-invalidate-partial-cache

FIX: Deliberately clear partial cache blocks on flush (fixes #1383)
API Enable reauthentication within the CMS if a user session is lost