Add flag to activate read only mode, fixes #5426.

sfBasicSecurityUser won't accept authenticated users if app_read_only is
activated. This setting accept values from two places: (1) app.yml, and (2)
the environment variable ATOM_READ_ONLY.

The string found in the environment variable is filtered as follows:
- TRUE if its value is "1", "true", "on" and "yes".
- FALSE otherwise.