Fifth and final piece of #591's initial implementation. Extends
/api/setup/version with update_available/latest_version/
latest_release_url (nil-safe via Server.UpdateCheckResult, defaults to
Available: false when the check was never enabled). Response switched from
map[string]string to map[string]interface{} to carry the new bool field;
updated the one existing test that decoded into the old stricter type.
Documents UPDATE_CHECK_ENABLED/UPDATE_CHECK_INTERVAL in the Configuration
Options reference table, explicit that this is the only network call
AfterTouch makes beyond speaker/provider traffic when enabled, and that it
defaults off.
This closes out the initial #591 implementation per the design doc
(_/i591/design-update-check.md): UpdateCheckState persistence, the
updatecheck.Checker package, background goroutine wiring with jitter/
backoff, reusing #419's Announcements mechanism instead of a second notice
UI, and this version-info exposure. `make check` passes end to end
(including the Docker HTTP integration suite).
Refs #591
Tobias Gesellchen
afaa00e483