Provide an option to generate a client token in the server UI

[mairas]

As discussed on Discord: while the auth token access request protocol provides a great user experience, it is somewhat involved from a developer's perspective. SensESP implementation, for example, is about 230 LoC in C++. To facilitate more light-weight client implementations, in a perfect world, it would be nice if you could generate new access tokens in the web UI.

To be clear, this is not needed for SensESP, but could be a useful feature for implementing new clients. A "nice to have" feature, that is.