Limits System and API

[timlinux]

We need a system for limits. Limits are quota's that define how many of a given resource an organisation (see #217) may have. Things that may be limited:

• Number of users for the organisation
• Number of projects for the organisation
• Number of and storage space occupied by related tables for the organisation
• Number of and storage space occupied by related indicator for the organisation
• Number of scheduled jobs for the organisation
• Which types of data importers can be used

By default, limits should be disabled for a deployment.
When limits are enabled, a new type of user 'platform manager' should be enabled. Platform managers can create or manage organisations and set limits.

The platform manager shall be able to issue themselves an API key that is used for managing the platform.

An API should allow a remote management interface to query and set the limits for a deployed instance.