Previously, ID for alert entity was generated without alertname or groupname.
This led to collision, when multiple alerting rules within the same group
producing same labelsets. E.g. expr: `sum(metric1) by (job) > 0` and
expr: `sum(metric2) by (job) > 0` could result into same labelset `job: "job"`.
The issue affects only UI and Web API parts of vmalert, because alert ID is used
only for displaying and finding active alerts. It does not affect state restore
procedure, since this label was added right before pushing to remote storage.
The change now adds all extra labels right after receiving response from the datasource.
And removes adding extra labels before pushing to remote storage.
Additionally, change introduces a new flag `Restored` which will be displayed in UI
for alerts which have been restored from remote storage on restart.
The source link is controlled by `external.url` and `external.alert.source`
flags, in the same way as for alertmanager notifications.
The source link is added to Alerts list view, and specific Alert view.
New UI pages:
/ - welcome page with API handlers list;
/groups - list of all rules per group;
/alerts - list of all active alerts;
/groupID/alertID/status - status of the active alert;
