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/common/migrations/0009_delete_currency.py

@@ -0,0 +1,18 @@
+# Generated by Django 3.0.7 on 2020-11-10 11:40
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('company', '0027_remove_supplierpricebreak_currency'),
+        ('part', '0057_remove_partsellpricebreak_currency'),
+        ('common', '0008_remove_inventreesetting_description'),
+    ]
+
+    operations = [
+        migrations.DeleteModel(
+            name='Currency',
+        ),
+    ]