Ftw-mail

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- Move functionality to create mailed-in mails to an adapter to allow
easier customization. [deiferni]

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- Fix setting defaults for choices with named vocabularies on inbound mails.
[phgross]

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- Mark inbound request with a marker interface.
[phgross]

- Drop Plone 4.1 support.
[phgross]

- Fix German and French FTI title of "ftw.mail.mail".
[mbaechtold]

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- Fix broken <meta /> tags in text/html MIME parts.
[lgraf]

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- Sort Mailtab so the newest Mail is the first Item.
[tschanzt]

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- Make upgrade step to fix `message.contentType` more defensive to avoid
trouble during upgrades caused by caching weirdness.
[lgraf]