Track current request by thread, without inspecting stack frames.
web hook plugin: Enqueue events as native types.

This ensures the task does not need database access.
Add get_base_url() in backend.

Obsoletes KegbotSite.{hostname, use_ssl}; see issue #273.
Drop `hostname` and `use_ssl` settings.

Issue #273.
backup: Add --no_media option.
upgrade: Add advanced options.