Add support for alignment on Group Block

[benlk]

This PR is a first pass at implementing something that Pam Dempsey asked about at lunch today, about whether it would be possible to have "sidebars" composed of multiple blocks, similar to something USA Today was doing in Seeds of Despair but pushed outside the main column of content.

Changes

This pull request makes the following changes:

• Copies the alignment controls for the core/group block from INN/umbrella-ipbs, but adding 'right' as an option
• Bumps Largo 0.6.4

This still needs:

• plan for dealing Bump Largo to latest version #32 with bumping Largo to 0.6.4:
  • What are the present CSS Customizer colors set to?
  • What happens if we disable the feature, bump Largo, then re-enable the feature?
• the Gruntfile changes from umbrella-ipbs
• proper alignment styles, rather than my slapdash -15vw
• padding if color?
• button styles within a colored group block
• better Editor support
• a decision about whether we want to keep the core/group alignment controls within child themes, or move them into Largo, or get them added to Gutenberg and eventually Wordpress. I think getting them into WordPress Core would be great.

Why

So that this code doesn't get lost, if Pam decides to move forward on this work.

For #

Testing/Questions

Features that this PR affects:

• post editor
• bumps Largo to 0.6.4

Questions that need to be answered before merging:

• Is this PR targeting the correct branch in this repository?
• ?

Steps to test this PR:

1. Add some Group blocks to a post, with pictures and so on. Align them left/right/center/none. Give them background colors.