Add a "Processing..." overlay to update iframes

[dd32]

On the back of #19781, i thought i'd raise this:

We should also investigate a iframe overlay that contains a "Working.. please wait" graphic that clears once the content of the iframe starts loading, This should help ease the pain of not being sure if the update process has started on servers that force gzip (and as a result, provide no user feedback until the process is complete) - See #18525 and #18239 for examlples of that

Fast partial build updates don't experience this problem, but branch-to-branch updates can take awhile on some hosts, it'd be a nicer user experience for those users affected by the above.