Return jsx for BackupGanttPlot label instead of format string

[phrdang]

Currently in BackupGanttPlot.jsx we have this code:

return `
      <strong>Duration</strong>: ${durationStr}<br/>
      <strong># of backups</strong>: ${numBackups}<br/>
      <strong>Start</strong>: ${startTimeStr}<br/>
      <strong>End</strong>: ${endTimeStr}
    `;

For this issue we want to return JSX instead of a format string just for better readability/maintainability/preventing typos.