Return attachment URLs with observation records

Description

Currently the client needs to make multiple calls to view observation attachments:

1. Get the observation
2. Get the URL for each attachment
3. Request the URL.

We could simplify things by removing step (2) and returning the observation with URLs included, something like:

interface Observation {
  attachments: Array<{
    name: string,
    ...
    urls: {
      original: string,
      preview?: string,
      thumbnail?: string,
  }>
}

@ErikSin @achou11 would this simplify the front-end implementation? I'm thinking of writing something generic for expanding references in data types, so that we can expand icon URLs and preset field references too. Could even make the getUrl methods internal if we end up not needing them.