Sort contracts by different criteria

[zarino]

Allow the list of contracts on the homepage to be sorted by any one of:

• [x] least time remaining (end time, ascending)
• [x] most time remaining (end time, descending)
• [x] longest duration (duration, descending)
• [x] shortest duration (duration, ascending)
• [x] highest total cost (cost, descending)
• [ ] highest estimated emissions (emissions, descending) – if we have that data

[struan]

Various of these need work to correctly handle null values and contracts that have ended.

[zarino]

We have most of this in place. Sorting by emissions estimates will be covered by https://github.com/mysociety/contract-countdown/issues/37.