Add a way to submit a "join form" to an API

[ethanjli]

We'll need to bring up an API for submitting registration information to the network controller's admin interface. We could just POST a JSON object to an HTTPS-secured endpoint with various possible human-input info fields (email address, device description, proposed name, etc.); the client should also automatically insert its node ID.

For now, the authenticity of all submitted information will need to be confirmed out-of-band before the join request is secure to approve.

This form could be used for requesting to have a join attempt be approved, and/or it could be used for requesting to elevate access to other devices over the network after joining the network.