Flag module permissions are not removed on uninstall

[jromine]

Flag module creates permissions 'flag flagname' and 'unflag flagname' for each flag created. When flag module is uninstalled, these permissions are not removed from any roles they were added to.

The problem is user module tries to remove these permissions after flag module is uninstalled and the list of flags is no longer available. See issue backdrop/backdrop-issues#810

The implemented work-around is for flag_permission() to check whether flag module is installed before querying its database tables to obtain a list of per-flag permissions.