Add support for changing the log level

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/config/conf_main.go

common/config/models_main.go

common/logging/logger.go

config.sample.yaml