Add 'currency' option for company

- e.g. an external supplier might have a default currency
- Adds a form input which only allows selection of allowed currency codes
- Add unit testing for currency validation

InvenTree/company/migrations/0029_company_currency.py

@@ -0,0 +1,19 @@
+# Generated by Django 3.0.7 on 2020-11-11 23:22
+
+import InvenTree.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('company', '0028_remove_supplierpricebreak_cost'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='company',
+            name='currency',
+            field=models.CharField(blank=True, help_text='Default currency used for this company', max_length=3, validators=[InvenTree.validators.validate_currency_code], verbose_name='Currency'),
+        ),
+    ]