reorganize /recordings JSON response

I want to start returning the pixel aspect ratio of each video sample
entry. It's silly to duplicate it for each returned recording, so
let's instead return a videoSampleEntryId and then put all the
information about each VSE once.

This change doesn't actually handle pixel aspect ratio server-side yet.
Most likely I'll require a new schema version for that, to store it as a
new column in the database. Codec-specific logic in the database layer
is awkward and I'd like to avoid it. I did a similar schema change to
add the rfc6381_codec.

I also adjusted ui-src/lib/models/Recording.js in a few ways:

* fixed a couple mismatches between its field name and the key defined
  in the API. Consistency aids understanding.
* dropped all the getters in favor of just setting the fields (with
  type annotations) as described here:
  https://google.github.io/styleguide/jsguide.html#features-classes-fields
* where the wire format used undefined (to save space), translate it to
  a more natural null or false.

ui-src/lib/support/RecordingFormatter.js

@@ -91,7 +91,7 @@ export default class RecordingFormatter {
         recording.videoSampleEntryWidth +
         'x' +
         recording.videoSampleEntryHeight,
-      frameRate: frameRateFmt.format(recording.frameCount / duration),
+      frameRate: frameRateFmt.format(recording.videoSamples / duration),
       size: sizeFmt.format(recording.sampleFileBytes / 1048576) + ' MB',
       rate:
         sizeFmt.format(recording.sampleFileBytes / duration * 0.000008) +