feat(web): service page, manual update, jobs list and live job log (#7)

Also publishes the jobs topic on agent revoke and updates the agent
snippet to mount stacks read-write at the same path.

internal/master/web/views.go

@@ -2,6 +2,7 @@ package web
 
 import (
 	"strings"
+	"time"
 
 	"github.com/shcizo/package-updater/internal/imageref"
 	"github.com/shcizo/package-updater/internal/master/model"
@@ -49,3 +50,15 @@ type agentRow struct {
 	model.Agent
 	Online bool
 }
+
+// jobDuration is how long a job ran, or has been running; "" before it starts.
+func jobDuration(j model.Job) string {
+	if j.StartedAt == nil {
+		return ""
+	}
+	end := time.Now()
+	if j.FinishedAt != nil {
+		end = *j.FinishedAt
+	}
+	return end.Sub(*j.StartedAt).Round(time.Second).String()
+}