* Adds a configurable 'reference pattern' to the IndexingReferenceMixin class
* Expand tests for reference_pattern validator:
- Prevent inclusion of illegal characters
- Prevent multiple groups of hash (#) characters
- Add unit tests
* Validator now checks for valid strftime formatter
* Adds build order reference pattern
* Adds function for creating a valid regex from the supplied pattern
- More unit tests
- Use it to validate BuildOrder reference field
* Refactoring the whole thing again - try using python string.format
* remove datetime-matcher from requirements.txt
* Add some more formatting helper functions
- Construct a regular expression from a format string
- Extract named values from a string, based on a format string
* Fix validator for build order reference field
* Adding unit tests for the new format string functionality
* Adds validation for reference fields
* Require the 'ref' format key as part of a valid reference pattern
* Extend format extraction to allow specification of integer groups
* Remove unused import
* Fix requirements
* Add method for generating the 'next' reference field for a model
* Fix function for generating next BuildOrder reference value
- A function is required as class methods cannot be used
- Simply wraps the existing class method
* Remove BUILDORDER_REFERENCE_REGEX setting
* Add unit test for build order reference field validation
* Adds unit testing for extracting integer values from a reference field
* Fix bugs from previous commit
* Add unit test for generation of default build order reference
* Add data migration for BuildOrder model
- Update reference field with old prefix
- Construct new pattern based on old prefix
* Adds unit test for data migration
- Check that the BuildOrder reference field is updated as expected
* Remove 'BUILDORDER_REFERENCE_PREFIX' setting
* Adds new setting for SalesOrder reference pattern
* Update method by which next reference value is generated
* Improved error handling in api_tester code
* Improve automated generation of order reference fields
- Handle potential errors
- Return previous reference if something goes wrong
* SalesOrder reference has now been updated also
- New reference pattern setting
- Updated default and validator for reference field
- Updated serializer and API
- Added unit tests
* Migrate the "PurchaseOrder" reference field to the new system
* Data migration for SalesOrder and PurchaseOrder reference fields
* Remove PURCHASEORDER_REFERENCE_PREFIX
* Remove references to SALESORDER_REFERENCE_PREFIX
* Re-add maximum value validation
* Bug fixes
* Improve algorithm for generating new reference
- Handle case where most recent reference does not conform to the reference pattern
* Fixes for 'order' unit tests
* Unit test fixes for order app
* More unit test fixes
* More unit test fixing
* Revert behaviour for "extract_int" clipping function
* Unit test value fix
* Prevent build order notification if we are importing records
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