[#8527] E-Mail attachment setting

[vellip]

Add new setting A4_EMAIL_ATTACHMENTS for when you need more than the email_logo.png attachment in your emails.

No breaking change as a fallback is included when the setting is not existing.

See liqd/a4-meinberlin#5908 for usage

Tasks

• PR name contains story or task reference
• Documentation (docs and inline)
• Tests (including n+1 and django_assert_num_queries where applicable)
• Changelog

[goapunk]

nice, thanks!