[WIP] Data importer

[SchrodingersGat]

This PR represents a major change in approach to how we import / export data.

Previously we have relied on external libraries for importing / exporting - which all come with various downsides.

The main idea of this PR is to utilize the existing DRF API framework for both importing and exporting of data. This should provide the following major advantages:

• Significantly reduce code duplication
• Hook in to existing API framework which is well tested
• Improve speed of import / export
• Allow finer control over import / export processes
• Allow specification of custom import hooks
• Expose import / export processes to custom plugins

Framework

Uses existing DRF serializers, just need to register with the @register_importer() decorator.

Related Issues

• Closes Data Import Wizard #6792
• Closes [FR] Import tool refactor #3101
• Closes Timeout when downloading large parts list. #6376
• Closes [Roadmap] Improve Data Export Functionality #6389
• Closes Exportable Allocate Stock to Build Table Data #6955
• Closes Consumed Stock Table Data #6956
• Closes Exporting a Stock table does not include the "Stock value" column #6321
• Closes Chosen currency is not selected for purchase orders, default currency is selected #6664
• Closes Purchase order doesn't find all components - and uses wrong currency #6647
• Closes Consumed Stock Table Data - Complete List of Desired Fields #7431
• Ref: Export of parts through admin fails with 502 Bad Gateway #5571

TODO

• Remove the APIDownloadMixin class (and all references)
• Remove the download_queryset method in a many existing views
• Use database model verbose_name if serializer does not provide a label attribute
• Add / update unit testing for new dataset export approach
• Add unit testing for new dataset import apprpoach
• Add worker task to cleanup old import / export sessions
• Add unit test for cleanup backup task
• Ensure user has correct permissions before creating a new import session
• Code coverage for additional code
• Ensure all 'required' fields are mapped before progressing
• Allow user to nagivate back to previous data steps
• Add data import section to "admin center"

Further Work

• Expose import / export functions to plugins
• Allow plugins to define custom export fields / processes
• Tag all "model resource" classes as deprecated and will be removed in further release
• Profiling for new data export functionality, to determine where we can optimize
• Allow bulk data export via django admin interface
• Allow data import via django admin interface
• Run data export in background worker
• Replace existing "BOM Import" tool with new importer
• Replace existing "BOM export" tool with new exporter
• Replace existing "part import" tool with new importer
• Replace existing "purchase order import" tool with new importer
• Remove django-import-export framework entirely
• Allow bulk "update" via import (override current records, not create new ones)