Users API: Implement search with database

[lnielsen]

Requires #1

Implement the service's search method using just the database layer (i.e. the datastore implemented in Invenio-accounts).

Service:

• Define service method
• Define service config
• Define links template
• Define a service schema
• Define a simple permission policy (can be used to block all methods not needed in the Records service).
• Define result object/list

Resource:

• Probably doesn't need to be customised.

Both /api/users and /api/groups should require an authenticated user to access.

The method is supposed to be rewritten once we're able to index users and groups in Elasticsearch, so no parameter parsing is needed.