Improvements for logging and testing

[timsavage]

• Feature flags now has a modify context manager that allows for patching flags
  that are reverted when the context manager exits (similar to settings).
• Add PyTest fixture patch_feature_flags that returns modify context for feature
  flags.
• Fully implement split logging configuration. Settings now contains LOG_HANDLERS
  and LOG_LOGGERS that are merged into the logging configuration before it is
  applied.
• Add a function settings_in_module in the testing package to fetch a list of
  setting names from a settings module.
• Injection Registry now has a modify context manager that allows for making changes to the registry for testing.
• Add PyTest fixture patch_injection that returns modify context for injection registry