mailman: remove DKIM headers

mailman alters content body by putting footers etc. and hence
invalidates DKIM signatures. Although they shouldn't theoretically hurt,
strip them when doing so to avoid confusion.

Change-Id: Ic8483364e81d44106a159b580929ef32409798f3
exim: add DKIM for wikimedia.org domains

This adds DKIM support to exim manifests with three different keys:
1) typical user/machine mails From: @wikimedia.org
2) wiki-mail, i.e. MediaWiki generated emails, also from: @wikimedia.org
3) lists.wikimedia.org mails

The first two share the domain with different selectors, while the third
one uses lists.wikimedia.org as the domain. This will effectively mean
that the wikimedia.org domain will soon have DKIM with *four* different
selectors: wikimedia, wiki-mail, fundraising (already exists), google
(for Google Apps originating mail, private key belonging to Google,
TBD).

The manifest could be a lot more DRY by having a separate ::dkim defined
type but this is left to a later iteration.

The private repo & DNS changes required by this change have already been
done; preexisting DKIM policy record specifies that *some* mail may be
signed for this domain, so this is effectively a no-op change.

DKIM support was added in exim4 4.70 and hence *cannot* be applied
manually to mchenry as it is right now.

Change-Id: I727cf3c894b99cff761c6822d719a755f7cd9cbc
Bug: 52569
RT: 5585