Show message when report/csv is downloading

[alice-hannah]

It can take a few seconds for the report in particular to download. It would be useful to display a message to the user to reassure them that something is happening.

Some ideas (but not sure how accessible):

• https://shiny.posit.co/r/articles/build/progress/
• https://stackoverflow.com/questions/17325521/r-shiny-display-loading-message-while-function-is-running