Reference fields (#3267)

* 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

InvenTree/build/migrations/0030_alter_build_reference.py

@@ -1,6 +1,6 @@
 # Generated by Django 3.2.4 on 2021-07-08 14:14
 
-import InvenTree.validators
+import build.validators
 import build.models
 from django.db import migrations, models
 
@@ -15,6 +15,6 @@ class Migration(migrations.Migration):
         migrations.AlterField(
             model_name='build',
             name='reference',
-            field=models.CharField(default=build.models.get_next_build_number, help_text='Build Order Reference', max_length=64, unique=True, validators=[InvenTree.validators.validate_build_order_reference], verbose_name='Reference'),
+            field=models.CharField(default=build.validators.generate_next_build_reference, help_text='Build Order Reference', max_length=64, unique=True, validators=[build.validators.validate_build_order_reference], verbose_name='Reference'),
         ),
     ]