Allow Almanac service types to define default properties

Summary:
Ref T5833. This allows Almanac ServiceTypes to define default properties for a service, which show up in the UI and are more easily editable.

Overall, this makes it much easier to make structured/usable/consistent service records: you can check a checkbox that says "prevent new allocations" instead of needing to know the meaning of a key.

Test Plan:

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T5833

Differential Revision: https://secure.phabricator.com/D10996