[BUU] Saving and success message tweaks

[mariocarabotta]

This card builds on top of scenarios 7 and 8 of #11059

Saving overlay

• blocking the table interface (but users will still be able to refresh or move to a different screen)
• loading should be always visible even if I have scrolled below the fold. see this issue for more details on current behaviour. Also note that this specific behaviour might be overlapping with sticky table header, and sticky "products have changes" container, described here

Designs here > https://www.figma.com/file/v1zbrWDZSRd3Nqoe0SJ2Sm/Engineering-Delivery---Back-Office?type=design&node-id=657%3A3131&mode=design&t=M5n7m73a4MSXR9jQ-1

Success message

• temporary message docked at the bottom center of the page
• can be dismissed by clicking the button
• will dismiss automatically after 4 seconds
• user can still interact with the screen interface

Designs here >> https://www.figma.com/file/v1zbrWDZSRd3Nqoe0SJ2Sm/Engineering-Delivery---Back-Office?type=design&node-id=658%3A3704&mode=design&t=M5n7m73a4MSXR9jQ-1
(Click ▶️ button at top-right to preview the workflow -- I had to log in for it to work)

[mariocarabotta]

The overlay spinner can just say "Loading" and be used across different contexts

[mariocarabotta]

when testing styles across different pages, please add any messages with the wrong style / type in the issue below and they will be updated separately

#11884

[dacook]

Discussed with Mario; we don't need a specific "Saving" status, the current "Loading" message is sufficient and perhaps even preferable.
So all done!