Incrementally send messages to project authors

[eljamm]

As an NGI team member, I want to send an email message to all project authors in several batches, but make sure that I don't send the same message twice to the same person.

Acceptance criteria

Given I have a complete list of project authors' contacts, when I send a message to some of them, they are added to the list of already contacted authors for that message.

Given I already sent a message to a certain author, when I try to send the same message to them again, the attempt gets rejected.

[fricklerhandwerk]

Related: #152 (comment)

[eljamm]

Currently, there is a bit of a difficulty with adding entries to the list of already-contacted authors in Notion since that requires that we modify the table's cell, which requires API-access.

A workaround for this is to include the message when importing entries in the Author email message tracker table, but this won't work if the author already exists in that table.