Add sort order to Membership Tier and Payment Plan

[dterkanian]

Membership Tier and Payment plan appear to be displayed in the order they were added, which can provide an inconsistent user experience.

Add a deterministic ordering to Membership Tier and Payment Plan for display and the admin console. Suggested ordering:

Membership Tier: order by name.

Payment Plan: order by Membership Tier, cost, then name.

Alternatively, add a field to the respective models allowing them to be explicitly ordered.