Allow Almanac Bindings to be enabled/disabled via API and support the "properties" attachment

Summary:
Depends on D19340. Ref T12414. Ref T13120. See T12414 for some discussion about direction here.

Since I think retaining "enabled/disabled" as a simple flag is reasonable, expose it via the API for readers and writers.

Also expose binding properties.

Test Plan:

• Searched for bindings and properties with "alamanc.binding.search".
• Enabled and disabled bindings with "almanac.binding.edit".

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13120, T12414

Differential Revision: https://secure.phabricator.com/D19341