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/migrations/0057_remove_partsellpricebreak_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 = [
+        ('part', '0056_auto_20201110_1125'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='partsellpricebreak',
+            name='currency',
+        ),
+    ]

InvenTree/part/migrations/0058_remove_partsellpricebreak_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 = [
+        ('part', '0057_remove_partsellpricebreak_currency'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='partsellpricebreak',
+            name='cost',
+        ),
+    ]