web, store, policy, wiki: Atom feeds for releases, commits, and an owner's activity !346

/{owner}/{repo}/releases.atom, /{owner}/{repo}/log.atom[/{ref}] and /{owner}/activity.atom serve Atom over the rows the pages already have. The owner feed covers public repositories only, since a reader carries no session; a private repository answers 404. Releases, log and owner pages carry the discovery <link>. Repository names ending in .atom are refused, like .git, so the owner feed cannot shadow one. Unit tests for the serialiser and event titles; e2e for all three feeds, content type, the private repository, and discovery links.

Closes #192