Summary:
- Adds a class ModelWithMetadata which models can now extend from
- Instances of this class can query metadata for a plugin via
`obj.metadataForPluginId(pluginId)`
- To observe changes on metadata it is sufficient to observe database changes on
the model. e.g.:
`DatabaseStore.findAll(Thread,
[Thread.attributes.pluginMetadata.contains(pluginId)])`
- To set metadata a new action has been created: Actions.setMetadata
- Adds a helper observable in nylas-observables to query for models with
metadata
- Merges CreateModelTask and UpdateModelTask into SyncbackModelTask
- Update SendDraftTask ans SynbackDraftTask to handle metadata changes
Test Plan: - Unit tests
Reviewers: drew, evan, bengotow
Reviewed By: evan
Differential Revision: https://phab.nylas.com/D2575
