Put in badge/page open for new features

[ianb]

When the extension is updated, users should get a "NEW" badge on the toolbar icon.

When the person next activates Firefox Voice, a new tab should open that shows the updates. This will be a page within the extension (design to be done in a different issue).

The latest version of the feature list should be kept in browser storage. Not every release will necessarily have updates (e.g., a bugfix-only release won't need a proactive release page), so we should explicitly say what version last had an update and test against that. (The current version can be fetched via getManifest().)

We may want to use a different icon instead of a badge.