QR code improvements

- Display QR codes as links to served images
- The qr_code plugin caches these images in the background
- Make a qr_code template to push out as a modal form
- Create a QRCodeView to simplify display of QR codes
- Add option to launchModalForm() to disable the 'submit' button

Refactored QR code display for

- StockLocation
- StockItem
- Part

InvenTree/templates/qr_code.html

@@ -0,0 +1,10 @@
+{% load qr_code %}
+
+<div class='container' style='width: 80%;'>
+    {% if qr_data %}
+    <img src="{% qr_url_from_text qr_data size='m' error_correction='q' %}" alt="QR Code">
+    {% else %}
+    <b>Error:</b><br>
+    {{ error_msg }}
+    {% endif %}
+</div>