Closes #29836

The agent diff toolbar item was causing the editor toolbar to show even
when all the other elements were disabled via settings.

This PR fixes this by setting the location to
`ToolbarItemLocation::Hidden` in the states where it shouldn't show.

It also adds a new a `toolbar.agent_review` setting to hide the agent
review buttons altogether. However, if the other toolbar elements are
hidden and the file isn't under review, the editor toolbar will still be
hidden. So you only need to set this to `false` if you don't want them
to show up even under agent review.

Release Notes:

- N/A