expose tenant booking limits in the API

[stakach]

We need a way of viewing the existing limits that have been set and possibly setting the new limits (independently of a full tenant update)

Thinking:

1. this request should pass back the current limits: /api/staff/v1/tenants method index should return booking_limits
2. have a request like GET /api/staff/v1/tenants/current_limits that returns the limits for the current domain that any user can make
3. have a request like GET /api/staff/v1/tenants/:tenant_id/limits that returns the limits for the requested tenant that any user can make
4. we should also have a request like POST /api/staff/v1/tenants/:tenant_id/limits that only sys-admins can make that sets the limits

So that we can build management UIs and expose limit numbers to frontends for frontend validation