Mobile: Show mover in block-toolbar on mobile

[jasmussen]

We need to surface the controls for up, down, trash and advanced on mobile. Let's also surface them on wide images.

Here's how it looks on desktop now:

On "wide" images we move the controls above the image. Let's do the same for the mover:

On mobile, let's try a new interface. Let's have an ellipsis on the right of the selected block:

When you click this ellipsis, a new toolbar expands and replaces the old one:

We can do this with simple responsive CSS. That is, we always have the mover, trash and cog present in the above-the-block toolbar, and simply hide these on desktop breakpoints, show them on mobile. I've checked the accessibility implications with @afercia, and so long as the controls are display: none; when they aren't used, they aren't announced by screenreaders or show up in the tab stack. On mobile, there is no tabbing, so the requirements here are different.

Older mockup can be seen here.

[jasmussen]

We should prioritize this one.

In case there isn't room for up/down arrows in the toolbar, we might want to address this one in conjunction with #706.

[jasmussen]

I updated the screenshots in this ticket with a new proposal, that should be easier to implement yet accomplish the same.

[afercia]

I'd just recommend a meaningful aria-label for the ellipsis button and an aria-expanded attribute to give users some feedback when they press the ellipsis button.