Refactor action links and buttons

Behavioural changes:
  * buttons
    * no 'do=show' but clean URLs (show and back buttons)
    * the alias 'subscription' for subscribe buttons is now available
  * links
    * draft link has class 'draft', not 'edit'
    * revisions link has class 'revs', not 'revisions'
  * both
    * 'revision' is added as an alias for 'history'

These changes were mainly introduced to reduce the differences between
links and buttons.