Add indentation to JSON payload in admin

It's pretty basic - some indentation and ordering only, pending some
super-whizzy JSON syntax highlighting widget. But at least it's not
just a single blob of text.
Add /static/ to gitignore

It gets populated when collectstatic is run on deployment, no
need to include in the repo.
Change CallbackEvent.foo_ properties to foo_name

I was originally using board_, card_, list_, member_, as a shorthand
for the related name values extracted from the payload, but there's
no obvious reason why, so reverting to the obvious version.