Draft: Introducing Messages data items on error tracking api

In this MR I introduce the message data item. This is still WIP but I would like to ask you to take a look already, just to make sure that you agree with the approach. If we agree then I will introduce unit tests. I will keep this MR scope limited to:

• Introduce migration scripts that are needed for messages
• Introduce all swagger changes for messages
• Implement POST /store and /envelope endpoints for messages
• Refactor code to increase readability. There are many more refactors that we can have. I am starting just with some basics. We can improve in following iterations.
• No implementation for the 3 endpoints for messages. For now we return nil.