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/management/commands/import_csv.py

@@ -0,0 +1,57 @@
+import sys
+from typing import Any
+from pathlib import Path
+
+from django.core.management.base import BaseCommand, CommandParser
+
+from todo.operations.csv_importer import CSVImporter
+
+
+class Command(BaseCommand):
+    help = """Import specifically formatted CSV file containing incoming tasks to be loaded.
+    For specfic format of inbound CSV, see data/import_example.csv.
+    For documentation on upsert logic and required fields, see README.md.
+    """
+
+    def add_arguments(self, parser: CommandParser) -> None:
+
+        parser.add_argument(
+            "-f", "--file", dest="file", default=None, help="File to to inbound CSV file."
+        )
+
+    def handle(self, *args: Any, **options: Any) -> None:
+        # Need a file to proceed
+        if not options.get("file"):
+            print("Sorry, we need a filename to work from.")
+            sys.exit(1)
+
+        filepath = Path(options["file"])
+
+        if not filepath.exists():
+            print(f"Sorry, couldn't find file: {filepath}")
+            sys.exit(1)
+
+        # Encoding "utf-8-sig" means "ignore byte order mark (BOM), which Excel inserts when saving CSVs."
+        with filepath.open(mode="r", encoding="utf-8-sig") as fileobj:
+            importer = CSVImporter()
+            results = importer.upsert(fileobj, as_string_obj=True)
+
+        # Report successes, failures and summaries
+        print()
+        if results["upserts"]:
+            for upsert_msg in results["upserts"]:
+                print(upsert_msg)
+
+        # Stored errors has the form:
+        # self.errors = [{3: ["Incorrect foo", "Non-existent bar"]}, {7: [...]}]
+        if results["errors"]:
+            for error_dict in results["errors"]:
+                for k, error_list in error_dict.items():
+                    print(f"\nSkipped CSV row {k}:")
+                    for msg in error_list:
+                        print(f"- {msg}")
+
+        print()
+        if results["summaries"]:
+            for summary_msg in results["summaries"]:
+                print(summary_msg)