webui: serve via nginx

Serve the mostly pre-compressed static webui content via nginx. If nginx
can't find a static file, proxy the URI back to the rest server.

A follow-up commit will remove code that serves webui from the
phosphor-rest-server.

Tested:
- The webui, rest server and bmcweb seem to be working as before.
- curl -H "Accept-Encoding: gzip" https://<witherspoon>/foo.html
  is served compressed content.
- curl https://<witherspoon

>/foo.html is served uncompressed
  content.

Change-Id: Ice443f00a467f3558f28d697261df4f23278f420
Signed-off-by: Deepak Kodihalli <dkodihal@in.ibm.com>