Reduce duplication in Makefile

- Install `gox` and `go-bindata` only when not installed
- Have only one target that specifies the asset bindata
- Build assets as normal dependencies

This also makes it easy to commit generated files as part of a pre-commit, e.g.

```sh
#!/bin/sh

git diff --cached --name-only | grep -qv '^static/' || (
        make build-assets && git add -f bindata.go && make build-dev-assets
)
```