The image block (and others) have an alignment toolbar that we need to implement

Edit: The theme-based options are now tracked via #1565.

There's a good amount of conversation about this in #169, but to summarize:

• The relatively easy part is to add the alignment toolbar and hint the selected setting by tweaking the image margins and alignment. This is what Aztec does already.
• If we want to wrap text around the image for left/right floats, that's much trickier and we're not worrying about that just yet.
• What alignment options are available depend on the theme, so we'll need to research how we'll get that information.

We'll want the first item for the beta (three days), but we might want to consider the other two depending on research.