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/templates/js/translated/api.js

@@ -62,12 +62,12 @@ function inventreeGet(url, filters={}, options={}) {
         url: url,
         type: 'GET',
         data: filters,
-        dataType: 'json',
-        contentType: 'application/json',
+        dataType: options.dataType || 'json',
+        contentType: options.contentType || 'application/json',
         async: (options.async == false) ? false : true,
-        success: function(response) {
+        success: function(response, status, xhr) {
             if (options.success) {
-                options.success(response);
+                options.success(response, status, xhr);
             }
         },
         error: function(xhr, ajaxOptions, thrownError) {

InvenTree/templates/js/translated/label.js

@@ -59,7 +59,6 @@ function selectLabel(labels, items, options={}) {
             </label>
             <div class='controls'>
                 <select id='id_plugin' class='select form-control' name='plugin'>
-                    <option value='' title='{% trans "Export to PDF" %}'>{% trans "Export to PDF" %}</option>
         `;
 
         plugins.forEach(function(plugin) {
@@ -207,19 +206,20 @@ function printLabels(options) {
                         href += `${options.key}=${item}&`;
                     });
 
-                    if (data.plugin) {
-                        href += `plugin=${data.plugin}`;
+                    href += `plugin=${data.plugin}`;
 
-                        inventreeGet(href, {}, {
-                            success: function(response) {
+                    inventreeGet(href, {}, {
+                        success: function(response) {
+                            if (response.file) {
+                                // Download the generated file
+                                window.open(response.file);
+                            } else {
                                 showMessage('{% trans "Labels sent to printer" %}', {
                                     style: 'success',
                                 });
                             }
-                        });
-                    } else {
-                        window.open(href);
-                    }
+                        }
+                    });
                 },
                 plural_name: options.plural_name,
                 singular_name: options.singular_name,