Store oob data into DB

Right now oob data is merged into the message text and stored into the DB.
If we load this message from the DB we cannot know anymore if and which
parts of the text was oob data.

Use the additional_data api to store oob data into the DB instead of
altering the original message text.

This lets us decide later on if and which data we want to display, even
on history replays. Also plugins can use the data.