Expose note.tags as list of strings

The tags field is currently exposed as a string field through the API. I think we should change it to be a list of strings even if the underlying ORM implementation still uses a CharField. This way, clients using the API don’t have to share a common delimiter when splitting the string into a list of strings.

If we want to change the ORM field that is used I suggest we use a JSONField, which is included in Django, has widespread database support and can be queried if that need should arise.