chore(housekeeping): Create helpers/change-detection.sh (#2610)

* chore: Extract change-detection method to it's own helper

This doesn't really belong in `helpers/ssl.sh`. Moving to it's own helper script.

* chore: Co-locate related change-detection method from container startup

It seems relevant to migrate the related support during startup for the change detection feature into this helper.

I opted to move the call from `start-mailserver.sh` into the `_setup` call at the end for a more explicit/visible location.

* chore: Move `CHKSUM_FILE` into `helpers/change-detection.sh`

It belongs there, not in `helpers/index.sh`.

* chore: Revise inline documentation

* tests(fix): Ensure correct functionality

Presently `test/test_helper.bats` is using it's own  `CHKSUM_FILE` instead of sourcing the var for the filepath.

`test_helper/common.bash` was calling a method to check for changes, but this helper may not correctly detect letsencrypt related changes as these are not ENV rely on, but global vars handled by `helpers/dns.sh`, so that should be run first like it is for `check-for-changes.sh`.

* tests(chore): Use `CHKSUM_FILE` var from helper

* chore: `addmailuser` should use `CHKSUM_FILE` var

* chore: Update `check-for-changes.sh` log message with correct path

target/scripts/startup/setup-stack.sh

@@ -7,6 +7,9 @@ function _setup
   do
     ${FUNC}
   done
+
+  # All startup modifications to configs should have taken place before calling this:
+  _prepare_for_change_detection
 }
 
 function _setup_supervisor
@@ -77,22 +80,6 @@ function _setup_file_permissions
   chmod 640 /var/log/mail/freshclam.log
 }
 
-function _setup_chksum_file
-{
-  _log 'debug' 'Setting up configuration checksum file'
-
-  if [[ -d /tmp/docker-mailserver ]]
-  then
-    _log 'trace' "Creating '${CHKSUM_FILE}'"
-    _monitored_files_checksums >"${CHKSUM_FILE}"
-  else
-    # We could just skip the file, but perhaps config can be added later?
-    # If so it must be processed by the check for changes script
-    _log 'trace' "Creating empty '${CHKSUM_FILE}' (no config)"
-    touch "${CHKSUM_FILE}"
-  fi
-}
-
 function _setup_mailname
 {
   _log 'debug' "Setting up mailname and creating '/etc/mailname'"