Improve the Inserter experience when editing pages in the Site Editor

[jameskoster]

There are a couple of issues it would be nice to address in the page edit experience in the site editor.

1. If you open the Inserter without first selecting the Post Content block it appears totally empty.
2. Dragging blocks from the Inserter to the canvas does nothing, despite the UI hinting that it will via cursor: copy.

For the first issue, I understand this is because blocks cannot be added to the template when you're page editing. But the empty Inserter is a bit unexpected. We might try adding a notice that explains the situation, or alternatively: display the blocks and add some logic so that they get added as child blocks of Post Content on click.

For the second issue, perhaps a modal could appear if you drop a block outside of Post Content, explaining why you cannot do that, and providing a link to go edit the template. Alternatively we could just use cursor: no-drop on incompatible regions — this would be more elegant, but potentially confusing ("Why can't I drop this block here?").

cc @SaxonF @noisysocks since y'all worked on this and may have additional context.

[jameskoster]

@richtabor potentially one for UX & Polish?

[SaxonF]

We have inserter behaviour as a follow up task here. I've linked it to this post for further context.

[jameskoster]

Inserter is no longer empty when editing a page in the site editor which was the main issue here. Closing.