Use arkindex-export in pdf export

Refs https://redmine.teklia.com/issues/3837
Depends #116 (closed)

We want to convert the SQL queries made in PDF export to arkindex-export.

1. Convert following methods to an ORM query directly set in the export code (no dedicated module)
   • list_folders
   • filter_folder_id
   • element_image
   • element_transcription
2. Same thing but using arkindex-export list_children to become recursive:
   • list_children
   • recursive_element_transcriptions

Do not remove any export methods yet.