Implement menu links as plugins, including static admin links and views, and custom links with menu_link_content entity, all managed via menu_ui module

This PR is just for interactive review since it's such a big changeset  - please summarize comments at: https://drupal.org/node/2256521
I may be wrong here, but I read somewhere in the phase1 or phase 2 issue that we can replace these menus by config change. No?