Include stock item purchase price in pricing cache (#4292)

* Add setting to control pricing calculation from stock items

* Bug fix for displaying a "null" setting

* Add new fields to PartPricing model

* Add code for calculation of min/max stock item costs

* Update pricing display to use stock item pricing

* Add unit testing for new pricing features

* Automatically update pricing when stock item is created or edited

* Increment API version

* Improvements for price rendering

* Update based on feedback:

- Roll stock item pricing into purchase pricing
- Simplify models
- Update unit tests

InvenTree/templates/InvenTree/settings/pricing.html

@@ -15,8 +15,11 @@
             {% include "InvenTree/settings/setting.html" with key="PART_BOM_USE_INTERNAL_PRICE" %}
             {% include "InvenTree/settings/setting.html" with key="PRICING_DECIMAL_PLACES" %}
             {% include "InvenTree/settings/setting.html" with key="PRICING_UPDATE_DAYS" icon='fa-calendar-alt' %}
+            <tr><td colspan='5'></td></tr>
             {% include "InvenTree/settings/setting.html" with key="PRICING_USE_SUPPLIER_PRICING" icon='fa-check-circle' %}
             {% include "InvenTree/settings/setting.html" with key="PRICING_PURCHASE_HISTORY_OVERRIDES_SUPPLIER" icon='fa-shopping-cart' %}
+            {% include "InvenTree/settings/setting.html" with key="PRICING_USE_STOCK_PRICING" icon='fa-boxes' %}
+            {% include "InvenTree/settings/setting.html" with key="PRICING_STOCK_ITEM_AGE_DAYS" icon='fa-calendar-alt' %}
             {% include "InvenTree/settings/setting.html" with key="PRICING_USE_VARIANT_PRICING" icon='fa-check-circle' %}
             {% include "InvenTree/settings/setting.html" with key="PRICING_ACTIVE_VARIANTS" %}
 
@@ -57,6 +60,7 @@
             <td></td>
             <th>{% trans "Base Currency" %}</th>
             <th>{{ base_currency }}</th>
+            <th colspan='2'></th>
         </tr>
         <tr>
             <td></td>
@@ -70,7 +74,6 @@
             <td></td>
             <td>{{ rate.currency }}</td>
             <td>{{ rate.value }}</td>
-            <td></td>
         </tr>
         {% endfor %}
     </tbody>

InvenTree/templates/InvenTree/settings/setting.html

@@ -27,14 +27,14 @@
         {% else %}
         <div id='setting-{{ setting.pk }}'>
             <span id='setting-value-{{ setting.pk }}-{{ setting.typ }}' fieldname='{{ setting.key.upper }}'>
-                {% if setting.value %}
+                {% if setting.value == '' %}
+                <em style='color: #855;'>{% trans "No value set" %}</em>
+                {% else %}
                 {% if setting.is_choice %}
                 <strong>{{ setting.as_choice }}</strong>
                 {% else %}
                 <strong>{{ setting.value }}</strong>
                 {% endif %}
-                {% else %}
-                <em style='color: #855;'>{% trans "No value set" %}</em>
                 {% endif %}
             </span>
             {{ setting.units }}