Improve terminology used

[philrandal]

Issue summary

MailWatch, like many other products, uses archaic and racially offensive terminology.

Terms such as blacklist and whitelist should be replaced with blocklist and allowlist as per the UK's NCSC's guidance:

https://www.ncsc.gov.uk/blog-post/terminology-its-not-black-and-white

c.f. the equivalent spamassassin bug report:

https://bz.apache.org/SpamAssassin/show_bug.cgi?id=7826