Add base URL option

[usmanmuhd]

The sensu web UI has to be hosted on https://abc.xyz/. Allow the UI to be hosted on https://abc.xyz/base_url/.

Feature Suggestion

In my use case, I'm hosting multiple applications on the same server and "/" is taken by other applications. Allowing the UI to be hosted on a different base_url would actually allow me to start using the same server to host multiple applications.

Possible Implementation

There is a public_url attribute under the webpack builder file. Need to make sure that is actually used in the react routing URLs.

[portertech]

Pull-request welcome, would merge.