Expose and refer to assets on the sub_uri.

When using sub-uri, create a symlink in public/ making the assets
available at the sub-uri. Modify environment.rb so that asset paths
refers to this sub-uri path. Closes http://bugs.joindiaspora.com/issues/391
and http://bugs.joindiaspora.com/issues/737 when application mounted
on suburi (i. e., prev patch)