Support SMTP article list import

[lentschi]

No description provided.

[lentschi]

see supported formats in shared_list:

@@file_formats ||= {
      'bnn' => ArticleImport::Bnn,
      'borkenstein' => ArticleImport::Borkenstein,
      'foodsoft' => ArticleImport::Foodsoft,
      'dnb_xml' => ArticleImport::DnbXml,
      'dnb_zip' => ArticleImport::DnbZip,
      'bioromeo' => ArticleImport::Bioromeo,
}.freeze

This can serve as a sample for re-implementation:
Embedding the gem in foodsoft (only applied to prototype fork):
https://git.local-it.org/Foodsoft/foodsoft/commit/5b75e655e923d10de60140975113ae58754293c9#diff-ca8e3c36c963e6d6cd8670a7e91b1d75353544c9

Gem:
https://git.local-it.org/Foodsoft/foodsoft_article_import