Remove old code for mailing list.

[brianjp93]

• Remove old templates, views, urls, tests and models
• Per #204, this project was abandoned.
• We will query and save aggregated stats from hyperkitty in a future commit, in #1289

[rbbeeston]

LGTM, also added Dave, as he's a better judge.

[vinniefalco]

Did the tables also get deleted? Where does the code do that?

[brianjp93]

@vinniefalco Yes, there is a migration file created by deleting the message model. https://github.com/boostorg/website-v2/pull/1363/files#diff-cf1b0c71fd0d510791428f09f54018a3389600f36dc90ab469f1f4e1263cf85e That migration file produces this sql.

BEGIN;
--
-- Delete model MailingListMessage
--
DROP TABLE "mailing_list_mailinglistmessage" CASCADE;
COMMIT;