Customization: Authentication

[bobbi-SMR]

Two problems in the documentation for creating a custom authentication handler:

• it buries the lead of where the handler should live (I had to read the docs more than once to find out the path);

• since it specifies that the path to the new handler should be backend/app/models/ . Since that path doesn't exist for standard installations, the documentation needs to specify whether the user can create that path, or has to go the "compile from source" route.