An empty file in one of the instrumented directories will cause the app
to fail to start when metrics are enabled. Metrics aren't enabled by
default in development or test.

We could handle the empty file case explicitly, but a file could still
not define the constant it is expected to, so instead run the
initializer manually in a spec and check that it succeeds.

If an environment variable exists for secret_key_base, use that -
always. But don't save it to secrets.yml.

Also ensure that we never write to secrets.yml if there's a non-blank
value there.

Move the last secret from .secret to config/secrets.yml, and delete
.secret if it exists.

.secret stores the secret token used for both encrypting login cookies
and for encrypting stored OTP secrets. We can't rotate this, because
that would invalidate all existing OTP secrets.

If the secret token is present in the .secret file or an environment
variable, save it as otp_key_base in secrets.yml. Now .secret can be
rotated without invalidating OTP secrets.

If the secret token isn't present (initial setup), then just generate a
separate otp_key_base and save in secrets.yml.

Update the docs to reflect that secrets.yml needs to be retained past
upgrades, but .secret doesn't.

Certain reverse proxies can send invalid IP addresses in the X-Forwarded-For header
For example, Apache can send (null).

Closes #20194

Storage path are not created until `TestEnv.init`, so we must skip
their validation on initialization.