Expand functionality of data export script

[olliestanley]

This refers to export.py

• [ ] add tree-state variable in root_object
• [ ] option to export all trees regardless of state or export trees in any specified state
• [ ] option to only export the prompts (only first message of tree)

This will include changing the function fetch_message_trees_ready_for_export in PromptRepository to take state as a parameter.

[andreaskoepf]

Ref to the existing fetch_message_trees_ready_for_export() function.