[META] Proof of concept

Problem/Motivation

As a start of D8 port, we should create a new D8 branch of Mailhandler. Info, module files, configuration, schema, install hooks. Create a minimal proof of concept. This will include creating new plugins for Inmail - a body-email handler that will be able to create a new node by filling the body field of a node from email body. This could require work on Inmail issues in order to get it into the state where it can be used well to integrate with Mailhandler.

Remaining tasks

• #2742555: Create info file
• #2742553: Add a composer.json file
• #2742559: Implement basic node handler
• #2742561: Add tests for node handler