Remove focused block on backspace

[aduth]

Related: #130

This pull request seeks to add basic backspace behavior for removing a block. We'll need to expand upon this, especially for blocks using the Editable component, as the block rendering cannot know when a block is empty (it should be up to a block to manage this consideration). As such, the changes here work best with blocks which do not auto-focus an Editable, e.g. the <hr> separator block. Blocks with editable text can be deleted by carefully clicking twice on the outer border of the block.

Testing instructions:

Verify that backspace deletes a block:

1. Select the separator block
2. Hit backspace
3. Note that the block is removed

Verify that backspace within a text region does not delete the block:

2. Select within a text block
3. Hit backspace
4. Note that the block is not removed

[youknowriad]

This makes me think, maybe we should revert this #530 since or maybe just make it obvious in the block API that it can be removed by backspacing the following text block.

[jasmussen]

I was thinking of this the other day. It seems like there are some fine interactions we need to polish.

#530 made sense, because you don't want to sit in a text block after an image block, backspace, and delete the image. But it still makes sense to let you select the image block, press backspace, and delete it. It also makes sense that backspace inside a paragraph block merges it with the previous block, as works now.

What if, when you are in a paragraph after an image and press backspace, first you select the image, and then if you press backspace again, you then delete it? We might need some changes that Ella proposed in a comment on #669 for this to work.

[aduth]

What if, when you are in a paragraph after an image and press backspace, first you select the image, and then if you press backspace again, you then delete it?

This makes sense to me.

That's consistent with the changes here too, yea? At least in the sense of assuming once the block has received focus.

[aduth]

I'm going to interpret the reactions as implied approval 😆

[jasmussen]

I'm going to interpret the reactions as implied approval 😆

Explicit approval!