Import tasks via CSV (#51)

* Bare start on CSV support

* Move core of CSV importer to operations

* More validations, break out validation function

* Validate dates and TaskList; convert errors to list of dictionaries

* Finish upsert code, and documentation

* Print msgs from the mgmt command, not the operations module

* Handle BOM marks

* Handle both in-memory and local file objects

* Update readme

* Working browser-upload view

* Bail on incorrect headers

* Fix default values and finish example spreadsheet

* Change column order, update docs

* Update index.md for RTD

* First round of responses to PR feedback

* Restore independent summaries/errors/upserts properties

* PR responses

* Split off reusable date validator into separate function

* Fix URLs append

* General test suite for CSV importer

todo/migrations/0009_priority_optional.py

@@ -0,0 +1,22 @@
+# Generated by Django 2.1.7 on 2019-03-18 23:14
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('todo', '0008_mail_tracker'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='task',
+            options={'ordering': ['priority', 'created_date']},
+        ),
+        migrations.AlterField(
+            model_name='task',
+            name='priority',
+            field=models.PositiveIntegerField(blank=True, null=True),
+        ),
+    ]