Should we create an .env file in the user home directory if we do not find one?

As of now, scancode.io checks to see if we have an `.env` file in `/etc/scancodeio/.env` or the root of the scancode.io directory. It would be convenient for scancode.io to look for an `.env` file in the user's home directory, especially in the case of running scancode.io from an AppImage (we cannot modify files within the AppImage).

We should:
1. Have an environment variable that points to an `.env` file to be used by scancode.io
2. Specifically for the AppImage context, create an envfile and populate it automatically with sane defaults for AppImage use

This file will live in the user's home directory and be named `.env-scancodeio` (or something to that effect). The secret key, DB file location, etc. should be generated automatically when first launching the AppImage.