fix bug with broken parsing of JSON patches

[mhl]

If you just reorder items in an 3 item array, then the JSON patch that is generated might be something like the following:

=======
op: add
path: /other_names/0
value: {u'note': u'', u'name': u'Joey', u'end_date': u'', u'start_date': u''}
=======
op: remove
path: /other_names/3

The idea is that these operations are applied in order; first the new item is inserted at the start, then the fourth item, at index 3, is removed.

However, the JSON patch isn't used like that in candidates/diffs.py - it tries to look up other_names/3 in the original data, which gives this error:

JsonPointerException: index '3' is out of bounds

Instead, it should apply the earlier operations before trying to parse the later ones.

This error is causing some person pages to error.

[mhl]

There's a failing test for this in the 873-broken-jsonpatch-parsing branch (PR here: https://github.com/mysociety/yournextrepresentative/pull/874)