[webapp] Add proxy suggestion/docs

[mwfarb]

To enable intentional control of webapp, the default access will be limited to localhost: netsec-ethz/scionlab#237. Users who still wish to access webapp remotely, should be given instructions to setup a proxy server to forward control and limit it as they wish.

• Add this warning with doc link to the webserver logs on startup.
• Add docs to suggest proxy uses for newcomers.