Migrate Note storage to Files

Note documents (schema note) are currently stored in String fields. This has several problems:

• Size is limited by the database storage, which causes errors on huge text files (for example MongoDB has a limit of 16MB).
• Strings don't record their original encoding; this causes round-tripping issues visible using Nuxeo Drive for instance.
• Huge intermediate String objects are created which may exhaust memory and cause OutOfMemoryError.

We should store Notes in the file schema and migrate old Notes.