* add new fields to Supplier Master:
- on_hold: To signal the Customer is blocked from completing certain transactions
- hold_type: 3 options - All, invoices and payments
* sanitize `on_hold` field input
* show hold status in list view
* add `release_date` field to Supplier Master:
- specifies the date when transaction restraint will be removed
* reset release date if supplier is not on hold
* add validation to stop transactions when Supplier is blocked
* add test cases
* return empty list for outstanding references if supplier is blocked
* block make button:payment if supplier is blocked
* adjust test cases
* PEP 8 clean up
* more tests
* adds new fields to Purchase Invoice:
- release_date: once set, invoice will be on hold until set date
- hold_comment: so user can add comment pertaining to why invoice is on hold
* implement individual purchase invoice on hold logic
* allow user to change release date
* update manual
* final cleanup including more validation and tests
* update supplier manual
* make default for release_date argument todays date
* remove Auto Repeat added by mistake
* add on_hold_field to purchase invoice
* add 'On Hold' or 'Temporarily on Hold' status for purchase invoice in list view
* implement explicit payment hold in purchase invoice
* update manual
* add dialog for saving comment
* bug fix, refactor, clean up
* more test cases
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