View linkerd dashboard at non-root URL

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## Bug Report

### What is the issue?
TLDR: Linkerd dashboard dependencies aren't using relative URL paths.

I would like to be able to have the linkerd dashboard running behind a reverse proxy at a URL like example.com/linkerd/. When I put linkerd behind a reverse proxy, the main page loads but can't find get the javascript dependencies retrieved via an absolute path.

### How can it be reproduced?
Add a k8s ingress that routes a non-root path to the dashboard. In this case, I'm using Traefik but Nginx, HA proxy, etc. would work.

Example ingress configuration:
```
apiVersion: extensions/v1beta1
kind: Ingress
metadata:
name: linkerd-dashboard-ingress
namespace: linkerd
annotations:
kubernetes.io/ingress.class: "traefik"
traefik.frontend.rule.type: PathPrefixStrip
spec:
rules:
- http:
paths:
- path: /linkerd
backend:
serviceName: linkerd-web
servicePort: 8084
```

### Logs, error output, etc
See image above.

#### `linkerd check` output

```text
$ linkerd check
kubernetes-api
--------------
√ can initialize the client
√ can query the Kubernetes API

kubernetes-version
------------------
√ is running the minimum Kubernetes API version
√ is running the minimum kubectl version

linkerd-config
--------------
√ control plane Namespace exists
√ control plane ClusterRoles exist
√ control plane ClusterRoleBindings exist
√ control plane ServiceAccounts exist
√ control plane CustomResourceDefinitions exist
√ control plane MutatingWebhookConfigurations exist
√ control plane ValidatingWebhookConfigurations exist
√ control plane PodSecurityPolicies exist

linkerd-existence
-----------------
√ 'linkerd-config' config map exists
√ heartbeat ServiceAccount exist
√ control plane replica sets are ready
√ no unschedulable pods
√ controller pod is running
√ can initialize the client
√ can query the control plane API

linkerd-api
-----------
√ control plane pods are ready
√ control plane self-check
√ [kubernetes] control plane can talk to Kubernetes
√ [prometheus] control plane can talk to Prometheus
√ no invalid service profiles

linkerd-version
---------------
√ can determine the latest version
√ cli is up-to-date

control-plane-version
---------------------
√ control plane is up-to-date
√ control plane and cli versions match

Status check results are √
```

### Environment

- Kubernetes Version: 1.15.3
- Cluster Environment: Bare metal
- Host OS: Ubuntu 16.04
- Linkerd version: stable-2.6.0

### Possible solution
Change dependencies to use relative URL.

Start by locating the Linkerd dashboard code that generates dependency URLs, then reproduce the issue with the provided Kubernetes Ingress configuration under /linkerd. Done means the dashboard loads its JavaScript dependencies and functions correctly when served at a non-root URL behind a reverse proxy.

Written by the indexing model from the issue text.