Add/Move 'Compare' button to operations column

Problem/Motivation

When displaying the 'node/XX/revisions' page, user does see two columns with radio buttons, and also the 'operations column' with revert/delete links.

For the user, it is unclear that another push button exists at the bottom of the page. Certainly when the node has mulitple revisions and the button moves 'below the fold'.

Proposed resolution

Add or Move the 'Compare selected revisions' button to the operations column.