[accesses] Dynamically generate accesses page

Changed the access_controller to dynamically generate the access json upon request.

The generator uses:

• accesses/nodesets.json from the refrepo output: a nodeset oriented representation of access rights with allowed ggas and sites for each priority level.
• /stable/users/ggas_and_sites from the UMS API: a list of all sites and ggas, with hierarchical information.

The generator:

1. expands all sites in a list of their respective ggas
2. Reverses the access information from a nodeset oriented view to a gga oriented one.
3. Completes accesses with missing no-access priority and the numerical access level information.