Introduce inbound email functionality

Summary
Customer is requesting that additionally to outbound email functionality, the APS product should also offer inbound email functionality out of the box.

Steps to reproduce
N/A

Business case
Customer would like to use inbound email functionality in order to

• start new process instances via inbound email
• complete activities via inbound email

Additional information
We already have some inbound email implementation that would need some review and could be ported into the product:
https://github.com/cijujoseph/activiti-examples/tree/master/aps-email-listener-extension