CSRF prevention for admin plugins

This patch adds a session based token to all form in the default action plugins.
The validity of the token is checked before any administrative function is
executed aiming to protect DokuWiki's admin functions from Cross-site request
forgery (CSRF) attacks.

Another patch will follow to add the same functionality on other, less critical
functions.

More details on CSRF attacks can be found at
http://en.wikipedia.org/wiki/Cross-site_request_forgery

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