Template vs content editing UX: improve block settings content to better indicate possible actions

[annezazu]

While doing some early testing of #49980, I noticed that it shows as “no block selected” in the sidebar when you're on the block tab:

no.block.selected.mov

It also doesn't provide any visual clarity in the sidebar when on the Page tab:

page.tab.mov

This feels like a missed opportunity, especially when the notice is hard to see on a dark background #51300 and only seems to appear once.

[noisysocks]

While doing some early testing of #49980, I noticed that it shows as “no block selected” in the sidebar when you're on the block tab:

I think the solution here is to mimic what happens in the other editors where we switch to the Page tab when you unselect a block. I'll work on a fix for that.

This feels like a missed opportunity, especially when the notice is hard to see on a dark background #51300 and only seems to appear once.

This notice will appear again now as of #51366.