Setup HTTP redirects using Google Cloud Traffic Management

Bazel's websites are behind two Google Cloud Load Balancers:

- `http-redirector` listening on `130.211.20.222:80`, `130.211.20.222:443` and `130.211.22.235:80`
- `bazel-build` listening on `130.211.22.235:443`

The `http-redirector` LB forwards all requests to an nginx instance running on a VM with this config:

```
http {
include /etc/nginx/mime.types;
default_type application/octet-stream;

# Redirect variations of the main website URL to the canonical one.
server {
listen 80;
server_name bazel.build www.bazel.build bazel.io www.bazel.io;
return 301 https://bazel.build$request_uri;
}

# Redirect http:// to https:// and *.bazel.io to *.bazel.build.
server {
listen 80;
server_name ~^(?.+)\.bazel\.(?.+)$;
return 301 https://$subdomain.bazel.build$request_uri;
}

# Redirect http(s)://ci.bazel.build/ to a new site.
server {
listen 80;
server_name ci.bazel.build;
return 301 https://github.com/bazelbuild/continuous-integration/blob/master/buildkite/README.md;
}

# Catch-all default server that just returns an error.
server {
listen 80 default_server;
server_name _;
add_header Content-Type text/plain;
return 200 "Bazel Redirection Service";
}
}
```

The full list of domains that are handled by the load balancers is:

```
bazel.foo -> 130.211.20.222
www.bazel.foo -> 130.211.22.235
tulsi.bazel.foo -> 130.211.22.235
ij.bazel.foo -> 130.211.22.235
skydoc.bazel.foo -> 130.211.22.235

bazel.io -> 130.211.20.222
www.bazel.io -> 130.211.20.222
tulsi.bazel.io -> 130.211.20.222
ij.bazel.io -> 130.211.20.222
skydoc.bazel.io -> 130.211.20.222

bazel.build -> 130.211.22.235
bcr.bazel.build -> 130.211.22.235
be.bazel.build -> 130.211.22.235
blog.bazel.build -> 130.211.22.235
ci.bazel.build -> 130.211.22.235
ci-staging.bazel.build -> 130.211.22.235
cr.bazel.build -> 130.211.22.235
cs.bazel.build -> 130.211.22.235
dashboard.bazel.build -> 130.211.22.235
docs.bazel.build -> 130.211.22.235
docs-staging.bazel.build -> 130.211.22.235
eclipse.bazel.build -> 130.211.22.235
ij.bazel.build -> 130.211.22.235
mirror.bazel.build -> 130.211.22.235
perf.bazel.build -> 130.211.22.235
releases.bazel.build -> 130.211.22.235
skydoc.bazel.build -> 130.211.22.235
tulsi.bazel.build -> 130.211.22.235
www.bazel.build -> 130.211.22.235
```

(For some unknown reason, the *.foo domains apparently do not work at the moment. 🤷🏻)

The idea behind this setup is that we want to do various kind of rewrites:
- Redirect old `*.io` domains to new `*.build` domains.
- Redirect `www.bazel.build` to `bazel.build`.
- Redirect `http://` URLs to `https://` URLs.
- Redirect `ci.bazel.build` to `https://github.com/bazelbuild/continuous-integration/blob/master/buildkite/README.md`.

The nginx VM was necessary because Google Cloud could not do these kind of rewrites for a long time. However, since 2020 a new feature called [Traffic Management](https://cloud.google.com/load-balancing/docs/https/traffic-management) is available and it looks like it should support this.

The goals of this project would be:
- Simplify our load-balancer configuration (ideally remove the `http-redirect` one completely).
- Shutdown and remove the `http-redir` VM.
- Implement the additional rewrite rule for https://github.com/bazelbuild/bazel/pull/13519: Redirect `https://docs.bazel.build/versions/master/...` to `https://docs.bazel.build/versions/main/...` (but only once documentation is being published under this new URL).