* app/vmalert: restore alerts state from datasource metrics
Vmalert will restore alerts state for rules that have `rule.For` > 0 from previously written timeseries via `remotewrite.url` flag.
* app/vmalert: mention remotewerite and remoteread configuration in README
vmalert: Extend web responses for alerts
* populate apiAlert object with additional fields
* return all active alerts, not only firing
* sort list of API alerts for deterministic output
* add helper for available path list