Export participants

• handle anonymous participants
• add icons in front-end
• check that dependencies are present in export
• expose enabled exports in config
• show only enabled exports in front-end
• check enabled exports in controller
• add a scheduled job to clean export files
• fix gitignore configuration to create needed folders
• create issue for documentation framasoft/joinmobilizon/documentation#8 (closed)

Closes #132 (closed)