Attachment metadata should only be store in doc.metadata_...

[charliepnnl]

Currently attachment metadata is being stored in two places:

• In `doc.metadata_.attachments[*]
• In the store state variable attachments[*].metadata

This is potentially confusing and error prone.
I would like to eliminate attachments[*].metadata.
This has several ramifications:

• The form of the attachments store state variable can be simplified to attachments = {[id]: blog, ...}.
  • Any code that uses the attachments variable will need to be updated accordingly.
• Any component wrapper that uses attachment metadata will need to pull metadata from StoreContext.Consumer.