Improved user permissions

REVU's permissions code will be rewritten to make it more modular. This will allow, for example, to give someone archive/unarchive/nuke privilegies without letting him advocate.

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Implementation Details:

We will use a number to represent the privilegies in the database, somewhat like chmod does.

Numbers available:
 - 1: Comment on packages from other people
 - 2: Archive/Unarchieve/Nuke packages
 - 4: Advocate packages

Possible combinations - Name they had before
0 - (Banned User)
1 - Contributor
3 - (Moderator)
5 - Reviewer
7 - Admin