Create properties on org model to process agreement info, adjust Agreement model with new details

[stvnrlly]

The second step of implementing ADR 0005

This is a collection of properties on the existing org model to pull in the data from the linked agreements.

• is_active should return whether the org has (1) an agreement with "signed" status and (2) we are within the period of performance
• agreement which just returns that active agreement or None
• has_mou should return whether the active agreement is an MOU (which would limit the org's services to the free tier only)
• remaining_budget calculates, well, the remaining budget based on sent messages and the cost of those messages. Note that there's trickiness here with the interaction with the free tier
• remaining_messages converts the remaining budget to remaining message fragments, based on our message rate

It's possible that those last two will require a bunch more thinking. If so, let's ship the first 3 and move the others to a new issue.

To complete:

• Add those properties
• Add tests for those properties

[xlorepdarkhelm]

I adjusted the models.py class Agreement to use two new Enum classes, AgreementType and AgreementStatus, which have options as specified in ADR-0005.

I also added the migration upgrade/downgrade for this.

[xlorepdarkhelm]

For less confusion, is_active is changed to be agreement_active