UI config changes

SingularityUI configs are all over the place and it should just share the index template that SingularityService uses. This PR aims to fix both of those issues.

#### Key points:

* SingularityService now uses the same index.mustache template as SingularityUI. A symlink points the SS one to SUI's one
* Everything else in SingularityService's file serving is the same. If you want to run Singularity completely locally, you have SS serve everything.
* If you want to run SingularityUI with a remote API, we need an `index.html`:
    * Every time Brunch builds SingularityUI, it generates an `index.html` with the settings it needs coming from Brunch's `config.coffee`. This is used by `brunch watch --server` exclusively.
    * When a user opens SingularityUI and there is no apiRoot set, they get a Vex prompt to enter one. As before, it uses localStorage so it can be edited at any time
    * `env.coffee` is completely gone. It wasn't really used anyway
* Updated UI docs

@tpetr @wsorenson 

might consider naming the local storage item `apiRootOverride`