R1:2f1b5ae01022

Give Almanac generic, custom-field-based properties

Summary:
Ref T5833. Currently, we have an `AlmanacDeviceProperty`, but it doesn't use CustomFields and is specific to devices. Make this more generic:

- Reuse most of the CustomField infrastructure (so we can eventually get easy support for nice editor UIs, etc).
- Make properties more generic so Services, Bindings and Devices can all have them.

The major difference between this implementation and existing CustomField implementations is that all…