record the user, who edits or adds an MQM-tag

The user, who added a MQM tag needs to be recorded according to MQM-syntax as specified by Arle Lommel, the user who adds the tag is added to the MQM-tag as saved with the segment in the database

• this user will be the user used for this tag when exporting MQM-tags with a CSV-export
• this user will be shown as a property of this tag
• in the segment metadata-panel, when the tag is selected for editing in the tooltip, when the user hovers over the tag