Add app tokens

[mbrodala]

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To deploy apps one needs to log into Nanobox on CLI. The same goes for CI systems if a deployment should be done after a successful build.

However I don't feel very comfortable exposing my Nanobox credentials in a CI system, no matter how secure it was built.

Thus I'd like to ask for app tokens. As the name indicates these can be generated per project (similar to the auth token per user) and can be used as an alternative method to log into Nanobox. These tokens could have additional restrictions (e.g. only allow for deploy).

It should also be possible to set an app token as environment variable (e.g. NANOBOX_APP_TOKEN) similar to #300.

[glinton]

@jedgalbraith any thoughts on this?

[jedgalbraith]

It's totally doable from a technical standpoint and wouldn't take much. I like it.