Fix loading of additional_data column from log database

Currently, the additional_data column is not correctly loaded from the log database in the logger module's methods get_last_conversation_lines and get_conversation_for_date. While the JSON data in the column is parsed, the parsed value is not saved, because the code assumes that changes to a loop variable are reflected in the list that is iterated over. Instead, the unparsed JSON string is returned. Fix this by building a separate list with the JSON string replaced by the parsed JSON object.

What gets me wondering is why that was not detected earlier. Obviously nobody is using that data. I looked around and it seems there is no user of the additonal_data field. Neither in the gajim nor in the gajim-plugins repository. The column was added (only) to the master branch around September last year. What is the plan for using this? Is it only used in an out-of-tree plugin somewhere?