* Add skeleton for builtin label printing plugin
* Force selection of plugin when printing labels
* Enhance LabelPrintingMixin class
- Add render_to_pdf method
- Add render_to_html method
* Enhance plugin mixin
- Add class attribute to select blocking or non-blocking printing
- Add render_to_png method
- Add default method for printing multiple labels
- Add method for offloding print job
* Simplify print_label background function
- All arguments now handled by specific plugin
* Simplify label printing API
- Simply pass data to the particular plugin
- Check result type
- Return result
* Updated sample plugin
* Working on client side code
* Cleanup
* Update sample plugin
* Add new model type
- LabelOutput model
- Stores generated label file to the database
- Makes available for download
* Update label printing plugin mixin
* Add background task to remove any old label outputs
* Open file if response contains filename
* Remove "default printer" option which does not specify a plugin
* Delete old labels after 5 days
* Remove debug statements
* Update API version
* Changed default behaviour to background printing
* Update label plugin mixin docs
* Provide default printer if none provided (legacy)
* Update unit test
* unit test updates
* Further fixes for unit tests
* unit test updates
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