Add "indexing in progress" banner to home page

[dsotirho-ucsc]

Hello, for your consideration:

User story

• As a viewer of the https://data.humancellatlas.org/ home page (before going to the /index/projects page)
• I want to be informed when a reindex is in progress
• so that I know the summary counts listed on the home page are temporarily low.

Definition of done

A banner or other visual indication is added to the https://data.humancellatlas.org/ home page when a reindex is in progress

[NoopDog]

Thanks, @danielsotirhos adding this to the backlog.

[bvizzier-ucsc]

Already done.