[D8] Convert "Recent content" block to a View

Followup to #40. Relevant drupal.org issue: https://drupal.org/node/2020393

The new views block should contain:

• [x] All the same fields as the node module recent content block
• [x] Operations links moved into a drop-button column
• [x] a setting for how many items to display (for BC funcionality)
• [x] a new setting for content types to display based on https://github.com/backdrop/backdrop-issues/issues/4020