clarify strategy for proxied resources

[mgrauer]

Based on comments in #241 from @jbeezley .

Currently we proxy WMS resources through cherrypy, #241 contains the relevant client and server side of this. We need a more scalable approach for this, but that requires documented guidance on the proxy server: nginx or apache or whatever else.

The reason we are proxying resources like non-authenticated WMS, and would like to proxy other tile layers (like open street map) is to serve resources under a unified scheme, i.e. all via http or all via https. When we serve Minerva behind https and some resources are served via http, the browser will prevent displaying the mixed content. This could be handled by a proxy server without going back to cherrypy.

For authenticated WMS layers, we need this unified scheme to avoid mixed content, but beyond that we need to proxy all the way back to cherrypy in order to decrypt the saved WMS credentials.

[mgrauer]

In addition to proxying WMS tiles, we need to proxy legends and getFeatureInfo requests.