Purpose
=======
Mailing lists are not flexible. It should be possible to merge mailing lists and remove duplicates when doing it.
Specification
=============
- When selecting records, in the action menu, add "Merge selected mailing lists"
- A modal will open :
- There's the choice between merging the contacts into a new mailing list or an existing one
- There's the choice to archive the source mailing lists or not
- The duplicates are avoided in the destination mailing list
- Opt-out contacts are not copied in the destination list
- Preserved explicit 3rd-party copyright notices
- Explicit boilerplate should not be necessary - copyright law applies
automatically in all countries thanks to Berne Convention + WTO rules,
and a reference to the applicable license is clear enough.