Writing Flow: Insert default block as provisional

[aduth]

Partially addresses #5396

This pull request seeks to change the behavior of the default block (both in the in-between inserter and end-of-content inserter) to remove the block if the user does not add any text or otherwise change the block before later deselecting the block.

Before	After

Testing instructions:

Verify that...

• You can insert paragraph blocks via enter and they are not removed upon deselection (e.g. pressing enter multiple times at the end of a post)
• You can add a default block (both in between-inserter and end-of-content inserter) and it is removed after deselecting without any changes
• You can add a default block (both in between-inserter and end-of-content inserter) and it is not removed if you make any changes to it before deselecting or saving

[jasmussen]

Love it. Ship it. 👍 👍 👍 👍

If we combine this with a plus icon somewhere, and perhaps reinstate that fancy resizing hit area you GIF demoed in a recent ticket, (separately from this), we'll have something.

Oh, and I owe you a beverage of your choice.