This change bundles the static assets like we have for v1 - using the [`embed`](https://pkg.go.dev/embed) go package. Fixes#22
In addition, it sets up a development script that runs `coderd` locally and serves the front-end, with hot-reloading. The script used is `./develop.sh`:
> NOTE: The UI is still placeholder, of course. Need to start testing out a simple, placeholder flow for the new v2 world as a next step
Summary of changes:
- Add build steps for `go` in the `Makefile`
- Add a step for production build, in which we use the `embed` tag
- Add a step for development, which doesn't need the `embed` tag - so we don't need to build the front-end twice
- Add `next export` build step to output front-end artifacts in `out`
- Add a `site` package for `go`
- Add `embed_static.go` and `embed.go`. This is mostly brought in as-is from v1, except removing some intercom/sentry CSP entries that we aren't using.
- Add a [next development server](https://nextjs.org/docs/advanced-features/custom-server)
- Add a `v2-dev` script, that runs `coderd` and the `next` dev server side-by-side
- Use the `site` package as the fallback handler.
- Add `.gitignore` entries for additional build collateral
