linkerd2 web does not support relative paths

Nobody has claimed this yet.

## Bug Report

I want to be able to proxy the web UI in a subdirectory or kubectl proxy, but it currently requires to be at the root path. That is a bug.


### What is the issue?

### How can it be reproduced?

### Logs, error output, etc

(If the output is long, please create a [gist](https://gist.github.com/) and
paste the link here.)

#### `linkerd check` output

```text
kubernetes-api: can initialize the client..................................[ok]
kubernetes-api: can query the Kubernetes API...............................[ok]
kubernetes-api: is running the minimum Kubernetes API version..............[ok]
linkerd-api: control plane namespace exists................................[ok]
linkerd-api: control plane pods are ready..................................[ok]
linkerd-api: can initialize the client.....................................[ok]
linkerd-api: can query the control plane API...............................[ok]
linkerd-api[kubernetes]: control plane can talk to Kubernetes..............[ok]
linkerd-api[prometheus]: control plane can talk to Prometheus..............[ok]
linkerd-api: no invalid service profiles...................................[ok]
linkerd-version: can determine the latest version..........................[ok]
linkerd-version: cli is up-to-date.........................................[ok]
linkerd-version: control plane is up-to-date...............................[ok]

Status check results are [ok]
```

### Environment

- Kubernetes Version:
- Cluster Environment: (GKE, AKS, kops, ...)
- Host OS:
- Linkerd version:
Client version: stable-2.1.0
Server version: stable-2.1.0

### Possible solution

Use a relative path of `dist` instead of `/dist` and so on.

### Additional context

Start at the linkerd2 web UI entry point and inspect how its dist paths are rooted when served through a subdirectory or kubectl proxy. Confirm the fix by loading the UI from a non-root path and verifying its assets and navigation work without requiring the root path.

Written by the indexing model from the issue text.