Enforce usage caps

[rossng]

At the moment, any conference using Clowdr can consume as many resources as it likes. We should implement automatically-enforced resource caps. For example:

• Maximum number of attendees
• Maximum livestreaming time
• Maximum size of uploaded content

[EdNutting]

• Maximum subscribed minutes in Vonage rooms
• Maximum number of organisers
• Automatic enforcement of the organising period, submission period, pre-publication period, pre-event period, event period and post-event period

[EdNutting]

Work is finally progressing on this