mh-junk: replace color-based terms with descriptive words

mh-junk: replace color-based terms with descriptive words

* lisp/mh-e/*.el: "whitelist" -> "allowlist" and "blacklist" -> "blocklist".
* doc/misc/mh-e.texi: update manual to match.
* lisp/mh-e/folder.el: Change the binding of 'mh-junk-allowlist' to 'J a'.
  Add a compatibility binding for the old 'J w'.
* lisp/mh-e/mh-scan.el (mh-note-allowlisted): Change char from 'W' to 'A'.
* lisp/mh-e/junk.el: Rename 'mh-blacklist-a-msg' to 'mh-junk-blocklist-a-msg',
  adding the missing "junk-" to the function name.