Pre-create allowance buckets for dimension-less resource grants

[scotwells]

After we completed the initial implementation of the quota system in #322, one common point of confusion was that a consumer's current quota limits couldn't be calculated until the first resource is claimed (or attempted to be claimed) from the system. This is a result of us doing lazy allowance bucket creation because of our long-term goals around supporting multi-dimension resource claiming.

To provide a more clear user-experience, we should pre-create dimension-less allowance buckets so that a user at least understands what their total consumption limit is, regardless of the dimensions that are used in claims. Additional allowance buckets may be created in the future as new claims are created with additional dimensions that hadn't previously had their limits calculated.

Relates to datum-cloud/enhancements#77

[scotwells]

Got a PR opened that introduces support for pre-creating dimensionless allowance buckets automatically when a ResourceGrant becomes active. This should enable us to show user's their baseline quota limits before a claim is first created.