Add an environment variable to allow app to be mounted below root

[rgtaylor]

The library app (as I understand the source code) assumes it is mounted at the root ('/') path of a website since all the redirects and auth callbacks start at '/' (e.g., '/login').

I would like to mount the app on a path instead. For example: https://www.example.com/wiki

This will be done using an NGINX reverse proxy mapped to the the '/wiki' location.

Ideally, there would be an optional environment variable that could be used to specify the mount point. LIBRARY_MOUNT_PATH='/wiki'