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/users/models.py

@@ -102,7 +102,6 @@ class RuleSet(models.Model):
 
         # Models which currently do not require permissions
         'common_colortheme',
-        'common_currency',
         'common_inventreesetting',
         'company_contact',
         'label_stockitemlabel',