This also adds a few improvements from the now deleted branch, #799, notably:

- clearing for video blocks. The responsive trickery means we have to, or we will get odd behavior otherwise
- better collapsing margins fix for the toolbar.

Update when sidebar is toggled.

There were some issues with centering blocks, due to the fact that it has a block mover on the left.

This commit simplifies that math, so the block mover doesn't actually affect the padding of the block, but rather uses negative space, making the math simpler.

Additionally, it adds some responsive fixes for the full wide toolbar positioning.

We are emulating floats using margins. This works well enough, but has some responsive implications. This commit addresses those, by creating a mix-in that simplifies the breakpoints.

I couldn't quite figure out where to best place this mixin, and we also probably want to extract both the mixin, the variables, and even the left/right alignment code so that other blocks that utilize these don't have to copy the whole bit of code. But I wanted to push this so we can test, and then discuss.

Clean up double semicolons, better center toolbars, remove border-box.

This commit hides the movers on floated images. We can look at reintroducing them for floats and full width images in a separate PR.

It also fixes another responsive issue where the sidebar is stickied.