luci-base: remove hardcoded cgi-bin path

Currently LuCI can be loaded only when placed in the root of the server as the cgi-bin path are hardcoded. Change the index.html to load the cgi-bin path from the current level.
Also add a new entry in the env Object to make the cgi_base path easily accessible.
This variable will be based on the position of /cgi-bin/luci.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>