dev_settings: Allow user to override settings in a git-ignored file.

This should make doing an ongoing development project related to a
settings variant significantly more convenient.

.gitignore

@@ -20,6 +20,7 @@
 /zproject/apns-dev.pem
 /zproject/apns-dev-key.p8
 /zproject/dev-secrets.conf
+/zproject/custom_dev_settings.py
 /tools/conf.ini
 /tools/custom_provision
 /tools/droplets/conf.ini

docs/subsystems/settings.md

@@ -110,6 +110,14 @@ additionally:
   You can see a full list with `git grep development_only=True`, or
   add additional settings of this form if needed.
 
+- If you need to override a setting in your development environment,
+  you can do so by creating a `zproject/custom_dev_settings.py`
+  setting the values you'd like to override (the test suites ignore
+  this file). This optional file is processed just after
+  `dev_settings.py` in `configured_settings.py`, so
+  `zproject/computed_settings.py` will correctly use your custom
+  settings when calculating any computed settings that depend on them.
+
 - `zproject/test_settings.py` imports everything from
   `zproject/settings.py` and `zproject/test_extra_settings.py`.
 

zproject/configured_settings.py

@@ -23,5 +23,21 @@ else:
     from .prod_settings_template import *  # noqa: F403 isort: skip
     from .dev_settings import *  # noqa: F403 isort: skip
 
+    # Support for local overrides to dev_settings.py is implemented here.
+    #
+    # We're careful to avoid those overrides applying to automated tests.
+    if not TEST_SUITE:  # nocoverage
+        import contextlib
+
+        with contextlib.suppress(ImportError):
+            from zproject.custom_dev_settings import *  # type: ignore[import, unused-ignore] # noqa: F403
+
+            # Print that we've got settings changes, so you know if you're testing non-base code.
+            #
+            # TODO: Figure out how to make this not be printed several
+            # times, and maybe print the actual keys that are
+            # overridden.
+            print("Using custom settings from zproject/custom_dev_settings.py.")
+
 # Do not add any code after these wildcard imports!  Add it to
 # computed_settings instead.