support multi tenancy

[cbellone]

Create a separate configuration for:

1. system
2. event
3. category

the settings should be applied in reverse order.
In order to differentiate events and avoid name clashing, the events should be named as user/eventName

[syjer]

Actually, it would be better to have the configuration at the following levels (add organization level):

1. system
2. organization
3. event
4. category

and the event should be on the organization/eventName url.

[syjer]

TODO:

• pass the most precise 'context' to the ConfigurationManager calls where possible
• implement the db changes (new tables ? modify the current configuration table?)
• implement the selection logic in ConfigurationManager
• tests
• implement admin GUI will be done in a next release
• check all the use of ConfigurationManager, check if the keys are all used consistently with the same "path".

notes:

• moving the events under org/event will be done in a separate task, there are too many changes involved.
• check the use of Configuration.system() . In some case we only need to find the related category/event/org/

[syjer]

The backend is done.