use liriae-pdf for submissions

• utils.py: add LiriaePdf, LiriaeFormConfig and related functions; remove open_issue and open_and_check_issue
• change output dir of uploaded pdfs to dossiers/...
• add tests/pdfs, tests/test_utils.py and enable pytest in CI
• modification.py: fix call to run_pdf_struct (pdf_path)
• Accueil.py: warning if videos not available
• merge: reorder widgets