privilege based access control

[nijel]

It should be possible to limit access to translating only to set of users. However the default setup should still be to allow everybody logged in to translate.

This requires:

• add set of privileges
• let various operations check whether user has this privilege
• add generic group Users, which will have all privileges
• make all new users be automatically part of this group

This basic setup would work as current system, allowing administrators to customize it (limit rights for Users group and adding separate groups for more fine grained access control).

[nijel]

Implemented in git.