Calculate status, delayed flag and decision date during data import

This took longer than I expected...

Pulls in decision date, works out status (Approved, Refused, In Progress) and attempts to calculate whether a decision is/was delayed from the planning hub data. (Approved/Refused requests may still be considered delayed if the decision was made after the stated target date.)

Passes the breakdown of Approved / Refused / In Progress through to the search controller (via a search facet). Filtering on that facet is available but once a filter is in effect, the breakdown is lost so in practice this may not be useful.

Performs the percentage calculations in the authorities#index view. Note - relies on councils supplying valid targetDate data (Rushmoor do), if they don't this may lead to underestimates which could unfairly favour councils that supply less complete data. May not become a problem but something to keep an eye on.

### Problems with current implementation

Some of the `/decision` records in the data hub are missing the `decisionIssued` element so although we can see that there is a `/decision` record, often with valid dates, we can't determine the outcome. There are currently 189 of these (out of 1137 records). Example references: `14/00539/FULPP`, `13/00222/CAT`, `14/00193/CATPP`  

(These are currently being flagged as "In Progress" for lack of a more useful description, they can be found in the database by pulling back all the records with a null `decision_date`)

Of these, there are 4 anomalies which also have a null `delayed` status as their `/decision` data contains neither a `decisionIssued` nor any form of `decisionDate`. Interestingly they all have council references ending in `PREAPP` which suggests that maybe they should be excluded somehow (although I doubt that the helpful PREAPP suffix will be consistent across all councils).

There are also 3 records which have a decision but no `decisionDate`, in these cases the `noticeDate` is being used instead as where both are available they appear to be identical. Applies to references: `13/00803/FULPP`, `14/00220/COND`, `14/00227/FUL`

### Room for improvement

At the moment the "stats" are just in the `Application` model. While this works well for displaying information on the results page - and doesn't appear to be too bad for the authority list - incremental updates could be problematic and it doesn't really lend itself to more complex calculations or future extensibility.

If we split the stats out into a separate table, we should consider whether we want to just hold all-time stats or have a more complex structure which will allow us to do monthly/annual/past *n* months cuts of the data.

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