Added the ability to archive projects:

* Only administrators can archive/unarchive projects.
* Once archived, the project is visible on the admin project listing only. It doesn't show up anywhere else in the app. Subprojects are also archived.
* Archive/unarchive preserve everything on the project (issues, members, ...).
* A subproject can not be unarchived if its parent project is archived.

git-svn-id: https://svn.redmine.org/redmine/trunk@549 e93f8b46-1217-0410-a6f0-8f06a7374b81