Hide block if being replaced

[mchowning]

Fixes #1177 by hiding a block that would be replaced when the add block bottom sheet is presented.

Further description and testing steps are available in the related gutenberg PR.

Update release notes:

• If there are user facing changes, I have added an item to RELEASE-NOTES.txt.

[marecar3]

Working as expected!
Nice work here! 🎉

The only thing that I would
probably change here is to put some prefix in front of the branch name, e.g. fix/hide_block_if_being_replaced or issue/hide_block_if_being_replaced as it's a good practice here.
But as that change would demand new PR, let's move it with this.

[hypest]

The only thing that I probably change here is to put some prefix in front of the branch name, e.g. fix/hide_block_if_being_replaced or issue/hide_block_if_being_replaced as it's a good practice here.

Good point. We technically follow the naming guidelines of the main WP apps anyway (see https://github.com/wordpress-mobile/WordPress-Android/wiki/Pull-Request-Guidelines) which point back to https://make.wordpress.org/mobile/handbook/general-guides/git-branching/#feature-branches. In this case, the typical branch name we'd use is: issue/1177-hide-block-if-being-replaced