Remove common_currency model entirely

- A lot of views / pages / etc needed to be updated too
- Now uses django-money fields entirely
- Create a manual rate exchange backend (needs more work!)

InvenTree/part/admin.py

@@ -279,7 +279,7 @@ class PartSellPriceBreakAdmin(admin.ModelAdmin):
     class Meta:
         model = PartSellPriceBreak
 
-    list_display = ('part', 'quantity', 'cost', 'currency')
+    list_display = ('part', 'quantity', 'price',)
 
 
 admin.site.register(Part, PartAdmin)