Label sheet printer (#5883)

* Add skeleton for new label sheet plugin

* Add custom printing options serializer

* Render individual label outputs to HTML

* Extract page size and column size

* Check label dimensions before printing

* Split labels into multiple pages / sheets

* Render out multiple labels onto a single sheet

* Cleanup base label template

- Allow @page style to *not* be generated
- Pass through as optional context variable
- Check that it still works for single label printing (default behaviour unchanged)
- Prevents multiple @page styles from being generated on label sheet output

* Fix stylesheets for part labels

* Cleanup stock location labels

* Cleanup more label templates

* Check if label can actually fit on page

* Generate output to PDF and return correct response

* Update panel.md

* Fix unit tests

* More unit test fixes

docs/docs/extend/plugins/panel.md

@@ -12,6 +12,7 @@ Most pages in the web interface support multiple panels, which are selected via
 {% include 'img.html' %}
 {% endwith %}
 
+
 Each plugin which implements this mixin can return zero or more custom panels for a particular page. The plugin can decide (at runtime) which panels it wishes to render. This determination can be made based on the page routing, the item being viewed, the particular user, or other considerations.
 
 ### Panel Content
@@ -311,4 +312,3 @@ import json
 The URL and the called function have no parameter names any longer. All data is in the
 request message and can be extracted from this using json.loads. If more data is needed
 just add it to the json container. No further changes are needed. It's really simple :-)
-