Add banner to announce websites

[Remi-Gau]

• set a banner on new website saying it is under construction and where to post feedbacks
• recycle the steering group election banner of the old website to announce the incoming new website with url and link to give feedback: https://github.com/bids-standard/bids-website/blob/gh-pages/_includes/steering_group_election_section.html
• add banners to the following websites adverstising that their content will be moved to the new BIDS website and with a link to the demo of the new bids website
  • starter kit
  • bids stats model
  • bids apps
  • bids execution
  • bids extension
  • ...