Update front-end for new party and person IDs

We've changed the scripts that create the basic PopIt instance so that:

* Parties use the Electoral Commission's party ID
* People have positive integer IDs

This requires (or suggests) a number of changes in the front-end:

* The list of available parties is now fixed; you shouldn't be able to create a
  party by adding an unknown party name into the party select box.
* We should only present parties from the Northern Ireland register in
  Northern Ireland, and similarly from the Great Britain register in Great
  Britain.
* All these parties are loaded from PopIt on start-up (this isn't ideal; see below)
* When a new person is added, they should be created with an integer ID

This pull request makes all of those changes, and in addition, it makes these
improvements:

* Javascript it loaded from files rather than being literally included in `<script>`
  blocks
* The static data loading is factored out into a new module; it was out of place in
  `models.py`
* Lots of now unused code is removed

There are two particular known bugs in this that should still be fixed:

* It's inefficient to load the parties on every start-up, and it's also difficult to mock
  this out in tests; instead, it would be better to have that data in the repository
  as with the MapIt data, and create a command to update that from PopIt. This
  could be run automatically whenever the party data in PopIt is updated.
* You get an error if you try to create a new candidate but don't select a party -
  I just haven't had time to fix this yet, but wanted to get the pull request out
  today.