message: fix mobile API doc nav for custom links
diff: {{- range $grouping, $names := $.Site.Data.packages }}
  <optgroup label="{{ $grouping }}">
  {{- range $names }}
  +
  + {{/* instead of a package name, we have a custom map used to create class/interface links */}}
  + {{- if reflect.IsMap .}}
  +
  + {{$custom := index . "custom"}}
  + {{ $customLink := index $custom "url" }}
  + {{ $customTitle := index $custom "title"}}
  + {{ $isCurrentPage := eq (relURL $current.RelPermalink) $customLink }}
  + <option value="{{ $customLink }}" {{ if $isCurrentPage }}selected{{end}}>
  + {{ $customTitle | humanize | title }}</option>
  + {{- else }}
  {{ $isCurrentPage := eq $package . }}
  <option value="{{.}}" {{ if $isCurrentPage }}selected{{end}}>{{ . }}</option>
  + {{- end }}
  {{ end }}
  </optgroup>
  {{- end }}
  