Apply code styles via pre-commit

- Add black and isort for Python styling
- Add doc8 for reStructuredText styling
- Add prettier for css/html/js/md/yaml styling
- Run all styling tools from pre-commit hooks
- Adjust flake8 config for compatibility with black
  (and current Django style)
- Add some other helpful pre-commit hooks
- Update editorconfig to match Django
  (with a few necessary adjustments for Anymail)
- Update `tox -e lint` to run all pre-commit hooks
- Update contributing docs

.flake8

@@ -1,2 +1,11 @@
 [flake8]
-max-line-length = 120
+extend-exclude =
+    build,
+    tests/test_settings/settings_*.py
+
+# Black compatibility:
+# - E203 (spaces around slice operators) is not PEP-8 compliant (and Black _is_)
+# - Black sometimes deliberately overruns max-line-length by a small amount
+#   (97 is Black's max-line-length of 88 + 10%)
+extend-ignore = E203
+max-line-length = 97