Rules engine for statuses

(See Alaveteli's-request-statuses for a list of the default statuses)

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

In order to support the different state machines for FOI in different jurisdictions, sooner or later we will probably need a rules engine, rather than trying to define configuration options in code.

One very flexible option is 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.