Only apply appender margins when the appender is inside of a block

[kjellr]

As pointed out by @jasmussen in #15868, the default appender for Twenty Nineteen appears improperly indented until it's clicked. After some investigation, I've determined that this is a side effect of the new(ish) margins added to the default appender in #14241.

In most themes, these margins are collapsed, so this has no effect. But they're visible in Twenty Nineteen, and push the placeholder text over:

This PR increases specificity of those margins so that they are put in place only when the default appender exists within an actual block. When it's outside of a block, those margins are unnecessary (I think?). This fixes the problem in Twenty Nineteen, and has no negative effects in any other theme I've tried.

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Before:

Twenty Nineteen:

Gutenberg Starter Theme (Appears as expected):

After:

Twenty Nineteen:

Gutenberg Starter Theme (No change):

[talldan]

I wonder how this issue regressed 🤔

The fix works well, and it'd be nice to get it merged, as its one of those things that's a fairly apparent visual issue—something that many will notice during their first experience of the editor.

Only other thought is that it might be worth adding a comment explaining the additional class.

[kjellr]

Only other thought is that it might be worth adding a comment explaining the additional class.

Thanks, @talldan! I've added an explainer in f74480e. I'll merge this once the tests pass.