Featureful settings. Add helper methods.

This change adds a couple of helpers and replaces some variables previously
referenced. PROJECT_ROOT still exists but should not be used, root() replaces
it.

We can now retrieve settings (in order of priorty) from end_user.py,
environmental variables, and a .secrets directory.

refs #14919
Extract out generating secret into function.

refs #14919
Update doc strings and comments.

refs #14919
Better doc strings for path helpers.

refs #14919
Update docs on configuring secret_key and api_key

Updated configuring.rst refs to settings, and added information about using
environmental variables or files to store the secret key values.

refs #14919
Add note about SECRET_KEY required length

refs #14919
Change wording on secret key and api key config docs

refs #14919
Changed version changed to version added.

refs #14919
Document env variable and file retrieval setting helpers

refs #14919
Merge branch 'bug/14919' into develop

* bug/14919:
  Document env variable and file retrieval setting helpers
  Changed version changed to version added.
  Change wording on secret key and api key config docs
  Add note about SECRET_KEY required length
  Update docs on configuring secret_key and api_key
  Better doc strings for path helpers.
  Update doc strings and comments.
  Extract out generating secret into function.
  Featureful settings. Add helper methods.