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

tox.ini

@@ -74,14 +74,21 @@ basepython = python3.8
 skip_install = true
 passenv =
     CONTINUOUS_INTEGRATION
-	# (but not any of the live test API keys)
+    # Make sure pre-commit can clone hook repos over ssh or http proxy.
+    # https://pre-commit.com/#usage-with-tox
+    SSH_AUTH_SOCK
+    http_proxy
+    https_proxy
+    no_proxy
+    # (but not any of the live test API keys)
 deps =
-    flake8
+    pre-commit
 commands_pre =
     python -VV
-    flake8 --version
+    pre-commit --version
 commands =
-    flake8
+    pre-commit validate-config
+    pre-commit run --all-files
 
 [testenv:docs]
 basepython = python3.8