Adjust Navigation Down bordering logic to avoid vertical margins multiplication when nesting

[jbinda]

Describe the bug
According to this comment the bug appears in Inner Block when nesting few Group block. Each level adds additional vertical margin.

To Reproduce
Steps to reproduce the behaviour:

1. Add new Group Block
2. Add Group Block inside freshly added Group block
3. Repeat step 2 as many times as you want
4. Notice each new Group block increases the vertical margins around placeholder in top most Group block

Expected behaviour
Vertical margin in top most Group block should not be affected no matter how many Group block will be nested.

Screenshots
If applicable, add screenshots to help explain your problem.

6 Group block nested vs 3 Group block nested

Smartphone (please complete the following information):

• Device: iPhone X
• OS: iOS 12.2, 13
• Version N/A

I think It affects all platform because it's JS issue in my opinion connected with the Navigation Down styling logic

Additional context
Please also refer this PR because it brings the styling logic when navigate throughout InnerBlock hierarchy. I think simple check to cut margin multiplications should solve the mentioned issue