Add an offline import tool for Synapse

CHANGELOG.md

cmd/export_synapse_for_import/main.go

cmd/gdpr_export/main.go

cmd/gdpr_import/main.go

cmd/import_synapse/main.go

cmd/media_repo/main.go

common/assets/process.go

controllers/data_controller/export_controller.go

controllers/data_controller/import_controller.go