Improvements to CSV generation

[mhl]

This PR introduces a number of improvements to CSV generation:

• The time taken to generate CSV files (tested on my laptop) has reduced from about 30 minutes to 3 minutes.
• CSV files that only include the successfully elected candidates are also generated. Fixes #911
• If a person has been a candidate for more than one post, they appear once for each post Fixes #891
• Useful extra columns are added to the CSV output.
• There are now tests that check that the number of queries required for CSV generation are constant per election.
• Sorting is improved; first names are also considered in sorting, and the sorting for the "all candidates" file is different from the election-specific files (alphabetical by name and grouped by posts respectively)

And an incidental improvement:

• Only a single query on ComplexPopoloField is used when accessing attributes of a PersonExtra.

[struan]

A few small comments aside this all seems to work and looks good.

I do note that if they remove some of the standard complex fields ( e.g. twitter_username ) then this crashes.

[mhl]

Thanks, @struan. I've created a ticket for the issue that CSV generation doesn't get its list of columns from SimplePopoloField / ComplexPopoloField. As we discussed in IRC, as well, I think it's best to put off dealing with this comment until after these changes are deployed.

[jf1]

@mhl Is is possible to also add a votes column to the all candidates csv?

[mhl]

@jf1 Once the PR is closed, could you create a new ticket for that kind of request? Otherwise it's likely to be overlooked.

[jf1]

Apologies, will do. Done.