feat(master): suggest newer version tags for third-party images (#11)

internal/master/web/templates/overview.html

@@ -77,6 +77,7 @@
     </div>
     <div class="flags">
       {{if .AvailableRef}}<span class="tag f-up" title="{{.AvailableRef}}">Update available<em>{{.AvailableTag}}</em></span>{{end}}
+      {{if .NewerTag}}<span class="tag f-newer" title="{{.NewerRef}}">Newer tag<em>{{.NewerTag}}</em></span>{{end}}
       {{if followsNoTag .Service}}<span class="tag f-notag">Follows no tag</span>{{end}}
       {{if .LastPollError}}<span class="tag f-err" title="{{.LastPollError}}">Poll failed</span>{{end}}
       {{if .Self}}<span class="tag f-out">Managed outside</span>{{end}}

internal/master/web/templates/service.html

@@ -9,6 +9,7 @@
   <tr><th>Status</th><td class="status status-{{.Service.Status}}">{{.Service.Status}}</td></tr>
   {{if .Service.DesiredRef}}<tr><th>Desired image</th><td><code>{{shortRef .Service.DesiredRef}}</code></td></tr>{{end}}
   {{if .Service.AvailableRef}}<tr><th>Available</th><td><code>{{.Service.AvailableRef}}</code> <span class="muted">via {{.Service.AvailableSource}}{{with .Service.AvailableAt}}, {{.Format "2006-01-02 15:04"}}{{end}}</span></td></tr>{{end}}
+  {{if .NewerTag}}<tr><th>Newer tag</th><td><code>{{.Service.NewerRef}}</code> <span class="muted">from the registry's tag list{{with .Service.NewerAt}}, {{.Format "2006-01-02 15:04"}}{{end}}; deploy it from Versions</span></td></tr>{{end}}
   <tr><th>Mode</th><td>{{.Service.UpdateMode}}</td></tr>
   <tr><th>Polling</th><td{{if .PollFailed}} class="error"{{end}}>{{.Polling}}</td></tr>
   {{if .ConfigFiles}}<tr><th>Config files</th><td>{{range .ConfigFiles}}<code>{{.}}</code><br>{{end}}</td></tr>{{end}}