Move like/share metaboxes to right side

[jaswrks]

Fixes #8924

Changes proposed:

Update Like & Sharing metaboxes:

• Use side context.
• Use default priority.

Testing instructions:

• Enable Sharing → Sharing Buttons
  • Confirm metabox shows on right side when adding a new Post.
• Enable Sharing → Like Buttons
  • Confirm metabox shows on right side when adding a new Post.
• Also test when one or the other is disabled.

Proposed changelog entry:

• Likes/Sharing: Move metabox in post editor to the right side for a better fit.

[designsimply]

Tested and confirmed using https://jurassic.ninja/create?jetpack-beta&nojetpack&branch=fix/like-share-metabox-location and by going to WP Admin > Jetpack > Settings > Sharing and enabling "Like buttons" (sharing buttons were on by default). Confirmed the in the meta box to either "Show likes" or "Show sharing buttons" disappears from the Posts > Add New page if either option is disabled and the whole meta box disappears if both are disabled.

^{Seen at https://convenient-cuban.jurassic.ninja/wp-admin/post-new.php running WordPress 4.9.4 and Jetpack 5.9-beta-17772-9255437-fix_like-share-metabox-location using Firefox 58.0.2 on macOS 10.13.3.}

[gravityrail]

LGTM

[mtias]

This also greatly improves the experience with Gutenberg: