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/check-for-changes.sh

@@ -36,7 +36,7 @@ fi
 # verify checksum file exists; must be prepared by start-mailserver.sh
 if [[ ! -f ${CHKSUM_FILE} ]]
 then
-  _exit_with_error "'/tmp/docker-mailserver/${CHKSUM_FILE}' is missing" 0
+  _exit_with_error "'${CHKSUM_FILE}' is missing" 0
 fi
 
 REGEX_NEVER_MATCH="(?\!)"