Reset current_week when new user is found in diary export

This should ensure that all headings are printed correctly for the weeks
of the diary.
Add all diary entries to export

Don't just add the text questions, add all questions and the answers.
Fix order_by call in ExportManager

Previously it was just ordering the whole query by week__week, so the
users were coming in a random order. By changing the call to just use 1
order_by and giving it 2 arguments, we can get the results correctly
ordered for the export.
Merge pull request #17 from mysociety/16-diary-export-data-missing

Diary export data missing