Blocks: Select the previous block when clicking backspace

[youknowriad]

This solves the second part of #666

When we hit backspace and we can't merge the block with the previous one, we select the previous one to be able to hit backspace a second time and delete it.

You can currently test it with the separator block followed by a text block (the image and the embed block have small focus issues right now, but there are open PRs to fix those)

[jasmussen]

Works really well. I dig this interaction! 👍

[aduth]

I pushed some adjustments, mainly in that if Editable's focus prop changes from true to false, it seems sensible that it blurs itself. With this I found we didn't need as much of the componentDidUpdate logic in block.js. In my testing this doesn't seem to interfere with Editable's assuming focus automatically when clicking the block.

If these changes make sense to you, feel free to merge 👍

Separately, I think another adjustment we should do is consider what happens after backspace deletes the block. Should we move focus to the previous block? We should consider the case someone wants to hold backspace to remove all content from their post.

[aduth]

The logic no longer exists, but this could have been expressed more simply as:

! this.node.contains( document.activeElement )

https://developer.mozilla.org/en-US/docs/Web/API/Node/contains

[youknowriad]

Separately, I think another adjustment we should do is consider what happens after backspace deletes the block. Should we move focus to the previous block?

I think this is a good thing. I added this interaction in 0d71661. How does that feel?

[jasmussen]

I think this is a good thing. I added this interaction in 5987b97. How does that feel?

Feels really really good. 👍