Scripts for finding accidental secrets in the repo.

This adds the following:
- A detect-secrets dependency and a related script
  (`script/detect_secrets`) to find and alert developers to secrets
  added to the code. By default, the script will search staged and new,
  unstaged files. It can optionally search only staged files.
- A whitelist, `.secrets.baseline`, that tracks instances of secrets or
  false positives already in the repo.
- Modifies `script/test` to detect secrets as part of the test suite.
- Updates to the README regarding the use of detect-secrets.

script/test

@@ -24,5 +24,8 @@ RUN_JS_TESTS="true"
 # Check python formatting
 source ./script/format check
 
+# Check for secrets
+./script/detect_secrets
+
 # Run the shared test script
 source ./script/include/run_test