Modify frontend to run from an arbitrary subdirectory

The frontend currently expects to be served by the top level domain. If this domain were `https://justincampbell.dev/`, the frontend assumed its relative path is `/`. However, in order to support [ a reverse proxy in production as described in this comment](https://github.itap.purdue.edu/ECN/webqueue2/issues/162#issuecomment-517), the frontend will need to be able to be served from a subdomain.

Ideally this configuration would come from a single point of configuration that is referenced throughout the project. [This article](https://scottvinkle.me/blogs/work/how-to-deploy-a-react-app-to-a-subdirectory) points out several different methods of achieving this but is somewhat out of date. Using it as a starting point, the frontend needs to be modified in the following ways:

#### 1: Add a `homepage` entry to the `package.json` file. 
This value can be referenced in code by the `process.env.PUBLIC_URL` variable. This can either for a full top level domain like `https://justincampbell.dev` or a path relative to the root of the top level domain. For example, if the frontend were to be served from `https://justincampbell.dev/cats`, the path relative to the root of the top level domain would be `/cats`.

Example:
```
# package.json
...
homepage: "/cats",
...
```

#### 2: Update non-client side routing.
Currently only API calls via `fetch()` are not client side routed. After a `homepage` entry has been added in step one, all `fetch()` requests to the API should be updated by using formatting strings and prepending the request path with the `process.env.PUBLIC_URL` variable:

```js
// Old Fetch
fetch("/login");

// New Login
fetch(`${process.env.PUBLIC_URL}/login`);
```

#### 3: Add a `basename` to React Router.
For all client side routing, we can set a `basename` attribute that prepends all Route and Link component paths. By using the `homepage` value from above as the `basename` we can have fully relative Links and Routes like this:

```jsx
// process.env.PUBLIC_URL = `/cats`
<Router basename={process.env.PUBLIC_URL}>
    <Link path="/page1"> {/* Full path is /cats/page1 */}
        Page 1
    </Link>
    <Link path="/page2"> {/* Full path is /cats/page2 */}
        Page 2
    </Link>

    <Route path="/page2"> {/* Matches path /cats/page2 */}
        Welcome to Page 2!
    </Route>
</Router>
```