Add a branch reference fixer as a pre-commit hook

- many *.py and *.ipynb files will contain links to resources on
  GitHub or nbviewer that have branch references in them
- add a pre-commit hook implemented as the nox session
  "fix-branch-references" that goes through these files and
  changes all the branch labels to the current one

setup.cfg

@@ -88,7 +88,7 @@ extend-ignore =
     B950,
     # Comply with black's style.
     # Source: https://github.com/psf/black/blob/master/docs/compatible_configs.md#flake8
-    E203, W503,
+    E203, W503, WPS348,
     # f-strings are ok.
     WPS305,
     # Classes should not have to specify a base class.
@@ -125,6 +125,8 @@ per-file-ignores =
         WPS213,
         # No overuse of string constants (e.g., '--version').
         WPS226,
+        # The noxfile is rather long => allow many noqa's.
+        WPS402,
     src/urban_meal_delivery/configuration.py:
         # Allow upper case class variables within classes.
         WPS115,