Alignment and padding controls for all blocks

[vsalvino]

Is your feature request related to a problem? Please describe.

Editors frequently need the ability to easily center align or adjust padding of content without using CSS. Considering this is a core part of most word processors and rich text editors (excluding wagtail), this is a big part missing from the editing experience.

Describe the solution you'd like

In the common block advanced settings, two toggles, one for alignment, and one for padding. These would work by simply adding the respective bootstrap classes based on user selection, similar to other advanced settings (e.g. column breakpoints).

Describe alternatives you've considered

The only real alternative is to add this only within rich text fields, or to add this to select blocks outside of advanced settings. However considering the ubiquity of alignment and padding, it makes sense to be added globally to all blocks.

Additional context

The default values of each should be unset, so as not to effect existing sites or default/global styles.

[vsalvino]

Moving this back to the backlog... it will end up being a bigger change and I'd rather get 0.19 out in a timely fashion to keep up with Wagtail updates.