refactor: Revise check-for-changes.sh

[polarathene]

Description

This PR is mostly housekeeping for check-for-changes.sh, in preparation for further refactoring.

Changes have been staged via individual commits with messages for further context. It may be easier to follow the changes via commit diffs, than as a whole.

Change Overview:

• Inline docs for check-for-changes.sh have been shuffled around and revised a bit.
• Change processing extracted from the main change detection loop method to their own methods:
  • _get_changed_files() - I extracted the grep+sed line to it's own method.
    • This was due to the verbosity of inline docs I added. I value being able to under to know quickly at a glance what I'm looking at without having to go refresh on the syntax 😅 - However if other maintainers feel this is a bad idea it can be reverted, no worries!
  • _postfix_dovecot_changes() - The bulk of change processing was moved to this method. I've added conditionals to only run relevant logic.
  • _ssl_changes() - Just shifted, no logic changed. REGEX_NEVER_MATCH and ACME_CERT_DIR vars scope set to local.

Type of change

• Improvement (non-breaking change that does improve existing functionality)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• New and existing unit tests pass locally with my changes

[georglauterbach]

LGTM 👍🏼