Label plugin refactor (#5251)

* 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

InvenTree/common/models.py

@@ -1794,7 +1794,7 @@ class InvenTreeSetting(BaseInvenTreeSetting):
 
 def label_printer_options():
     """Build a list of available label printer options."""
-    printers = [('', _('No Printer (Export to PDF)'))]
+    printers = []
     label_printer_plugins = registry.with_mixin('labels')
     if label_printer_plugins:
         printers.extend([(p.slug, p.name + ' - ' + p.human_name) for p in label_printer_plugins])