Pathname prefixes should be set automatically when DASH_APP_NAME is present #164

The logic for encoding DASH_ROUTES_PATHNAME_PREFIX and DASH_REQUESTS_PATHNAME_PREFIX should be refactored to set these based on available information as follows:

• check if routes_pathname_prefix or requests_pathname_prefix has been passed to Dash directly, and use if available
• check if DASH_ROUTES_PATHNAME_PREFIX or DASH_REQUESTS_PATHNAME_PREFIX have been set in current environment, and use if available
• check if DASH_APP_NAME has been set in environment, and use if available

This change would obviate the need to provide conditional logic in the header of apps for deployments to Dash Enterprise. since these prefixes must currently be set explicitly.