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/company/migrations/0027_remove_supplierpricebreak_currency.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.0.7 on 2020-11-10 11:40
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('company', '0026_auto_20201110_1011'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='supplierpricebreak',
+            name='currency',
+        ),
+    ]

InvenTree/company/migrations/0028_remove_supplierpricebreak_cost.py

@@ -0,0 +1,17 @@
+# Generated by Django 3.0.7 on 2020-11-10 11:42
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('company', '0027_remove_supplierpricebreak_currency'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='supplierpricebreak',
+            name='cost',
+        ),
+    ]