Tool to compare dashboards across environments

Assumes staging and preview currently, pass the slug as an argument and
it will request both and diff them.
Lookup dashboards by slug

- Gives us nicer urls like /edit/name-of-dashboard
- Slug is also a unique identifier
- Avoids exposing uuid to end user

I prefer this way of looking things up because it's kind of human readable but at the same time works for the computer. UUIDs are still really useful for how we internally manage resources. For example we could potentially look up the history of a resource whose `slug` has changed multiple times, but the `UUID` will have remained the same.
Change `spotlight_config_migration.py` to use dashboard slug

- (✿◠‿◠) Tried and tested with today's database! (●ω●✿)
Accept either slug or UUID to return dashboard resources

- A bit fugly in the views because we're doing stuff like `is_uuid(identifier)`
- Allows us to do e.g. `/dashboard/{uuid}` and `/dashboard/{slug}`
- Could do with a refactor but I think that can probably be bundled up with https://www.pivotaltracker.com/story/show/78992766
Add `test_list_modules_by_uuid_or_slug` test

- Show that you get resources back by slug or uuid
Merge pull request #255 from alphagov/get_by_slug_or_uuid

Get dashboard by slug or UUID
Return errors as per JSON API suggestion

See http://jsonapi.org/format/#errors
Merge pull request #230 from alphagov/feature/json-api-errors

Return errors as per JSON API suggestion
Merge pull request #254 from alphagov/add-compare-dashboards-tool

Tool to compare dashboards across environments
wip - dir for schema import which will allow common command on deploy
wip start of script, need to generate first. check how used in spotlight...

also add in modules_json generated from spotlight
wip remove old schemas - this is js

add way to run and proper loading too
wip getting there with the real schemas
Working schema updater

What's the deal with dashboards.json? looks like we have nothing
analogous in the whole project. Also, the query formatter is a bit like
the root module schemas but everything is optional? And there are some
other things?

All odd