Rules engine for statuses

The logic on how to automatically classify requests over time is current hard-coded in the [[InfoRequest model|https://github.com/sebbacon/alaveteli/blob/master/app/models/info_request.rb]]

For defining rules for states between which FOI requests can mcove, sooner or later we will probably need a rules engine, rather than trying to define configuration options in code.

An option requiring less system knowledge from the administrator would be to base it all around search queries:

Alternatively, we could parameterise it, to make it easier and less error-prone (at a cost of some flexibility):

Of course, an option combining the two (simple and advanced) is also possible.