simplify menu admin

| Q             | A
| ------------- | ---
| Bug fix?      | usability bugfix
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This PR aims to simplify the admin by removing the additional tab "Menu" that just contained things that a user does not want to edit, and the locale chooser which was basically in the wrong tab. Instead of calling the tab i kept "Menu Root" i chose to go back to "General" so that this admin is as similar to the normal node admin as possible.

/cc @dantleech - would be awesome if you have time to review today, and this could make it into the next tag