Feature change notifications should list fields by human-readable name, not field identifier

[jrobbins]

Currently our feature change notifications list the fields that were changed by naming field identifiers. E.g.,

Changes:

activation_risks:
old: Polyfills could be written to implement most of the functionality. But since most applications use libraries instead of direct WebRTC APIs, they probably won't benefit from a polyfill directly.
new:

wpt:
old: False
new: True

It should show human-readable names for these fields. E.g.,

Changes:

Activation Risks:
old: Polyfills could be written to implement most of the functionality. But since most applications use libraries instead of direct WebRTC APIs, they probably won't benefit from a polyfill directly.
new:

Web Platform Tests:
old: False
new: True

[jrobbins]

This might require moving the content of client-src/elements/form-field-specs.js to a python file and adding a request handler to serve it as JSON. That way the field ID to field name mapping would be available in both python and JS. It could be a little tricky because there are a few different files of JS constants that might all be better as Py constants.