Using the node red page through the tunnel

[bewee]

Currently, the node red page is only available when accessing the gateway locally, i.e. not via the tunnel. That's because the node red page is simply included via an iframe.

Making it available through the tunnel would require:

• somehow redirecting all traffic between the node red web app and the node red server to an apihandler, which would simply forward to the "real" local webthings server
• the node red plugin could no longer talk to the gateway directly, but would also have to take an extra indirection: Save accessToken as credentials node-red-contrib-webthingsio#2