EBI-wide user survey - add banner to website

[ljwh2]

EBI-wide user survey goes live on 20th May, we need to add a banner to the homepage as described here, and remove in mid June (date TBC):

https://docs.google.com/document/d/197gXfPhcJR5cIzUvWjC8ZKecUzUe6pbjgK61xOmVb-g/edit#heading=h.lu0yeii3ixvd