Release v6.6.0

• [change-log]
• Period Closing Voucher Test Cases
• Period Closing Voucher as per multi currency
• [fix] For Serial No Status
• [minor] leave application cleanup
• minor improvement in patch
• [fix] test case for purchase invoice gle
• [fix][patch] Fix wrong gle for Purchase Invoice against Expenses Included in Valuation account
• [translations] updated
• [language] Added Ukranian language - українська. Closes frappe/translator#57
• [fix] variant website page render
• [enhancement] Quick Entry in Journal Entry, remember Posting Date
• [feature] rename via console merge with frappe/frappe#1349
• Prevent Template Item from going into infinite loop while saving
• [minor] added column type definition in batch-wise balance history
• [fix] email digest cache
• [fix] re-run thumbnail patch
• [fix] ignore mandatory when creating customer and contact in cart
• [minor] don't display missing file doc message in item's make thumbnail