Buttons block: centered buttons are pushed to the left

[Aljullu]

To reproduce

1. Create a page and add the Buttons block.
2. Add a button and center it.
3. Notice it has a right margin, so it's actually not completely centered in the page.

It's easier to notice if you add an element above or below which is also centered (see screenshot below).

Expected behavior
We should remove the margin from the last button, so buttons are really centered.

Screenshots