Don't error if new fields are now expected
Fix tests to account for the "double PUT" workaround in 10890a694

These tests should have been updated in that commit, but they've been
pushed to master for some time now.
Make sure the party_memberships field is actually written back to PopIt

When switching to doing the PopIt updates in update.py, I accidentally
left out the party_memberships field, and only included standing_in.
This doesn't actually make a difference to the running of the site (since
party membership data is in memberships anyway), but it's helpful to
people getting data from the API to have a simplified representation.
Add a gender field for people
Fix the ID of the Labour Party in an example in a comment
Add 'post_id' to the 'standing_in' documentation
Remove an unused function
Allow addition query parameters to be specified to get_search_url
Give some variables better names
Make sure 'identifiers' can be set and preserved

This is important because we use it to store the old YourNextMP ID from
the imported data.
Add a comment (hopefully?) explaining get_person_data_from_dict
Add a field for the party PPC page's URL

These have quite a bit of information on them, and they're typically the
source for our PPC data, so we should make them available.
Add manually matched PPCs
Add a command for importing PPC data

If it thinks it's found a match for the PPC, it'll ask the user whether
they're actually the same, and record that decision so it doesn't have
to ask again.