Allow 'Standing in 2015?' to be Yes/No/Not Sure, not just Yes/No

Fixes #43
Add a FIXME suggesting some refactoring
Use party / constituency data from 2010 as defaults (if available)

This means that if you edit a person who's not yet marked as standing in
2015, but for whom there is default data from 2010, that 2010 party and
constituency will be prepopulated in the party / constituency box when
they're unhidden.
Change page title on leaderboard, fixes issue #81
Merge branch 'standing-in-not-sure'
A command to dump the live PopIt instance in a mongimport-able format
Don't include people who've changed constituency in the 'not sure' section

Previously, people would only have been excluded from the "not sure"
section on the constituency page if either they're marked as standing
again in that constituency, or they're marked as not standing anywhere -
it was missing people who we know are standing in a different
constituency.

Fixes #88
Add the jsonpatch package
Add a function to supplement versions with diffs between them

This uses jsonpatch to generate a patch we can render like a diff.
Render difference between versions, not just the whole version

This also hides the full version data unless you click 'Show'

This is a bit ugly, but we should have some designer time soon to tidy
it all up.

Fixes #39
Merge branch 'show-version-diffs'
Don't output adding a falsy value
Extract operation['op'] to a variable
Don't output changing a field between null and ''
Strip the leading '/' from jsonpointers for output
When replacing no value with some data, call it an "add"
Add a missing closing parenthesis
Display human-readable diffs of standing_in and party_memberships
Revert "Don't allow @parliament.uk email addresses"

This reverts commit d89df41e0546d6c1c8bc43df0ef9a1e15a641a78.

I'm reverting this because it means that you can't edit an existing
person with a parliament.uk and save them without deleting or replacing
that email address - this turns out to be a pain, so I think we'd be
better off going with one of Tim Green's suggestions: some time before the
dissolution of parliament sending a bulk mail to remaining parliament.uk
email addresses asking for an alternative.  See the discussion in #63.

Conflicts:
	candidates/tests/test_validators.py
Make sure the PPC importer doesn't overwrite existing data