Simplify user management configuration by integrating it into Confluence administration area

Right now it is very dangerous and a possible cause for a lot of additional support time for Atlassian to allow customers to upgrade osuser schema to atlassian-user, because the method that is required to do this here indicates you should overwrite atlassian-user.xml.

The problem with this method or even adding the hibernate element to the config manually is that whenever you update to the latest confluence war you could very easily forget about this configuration.

Outside of the database connection information itself (which is not stored in atlassian-user.xml), there doesn't seem to be any reason to keep this information in a local config file when it could easily be stored in the confluence database. At the very least it would seem that this config file should be stored in confluence.home dir to keep it from being overwritten during an upgrade. You could take this so far as to say that any customer-specific configuration should be either stored in confluence.home or in the database to avoid issues when customers are upgrading to a later version of confluence.