point comm link towards per-app dashboards (bug 928160), expose version #s and links to related threads (bug 928165)

Related to https://github.com/mozilla/commbadge/pull/25

API changes.

**928160**

- Point commbadge link to new commbadge route (/comm/app/APP_SLUG).
- Add app_slug to addonserializer
- Change commbadge webapp queryset to be ```.all()``` so that we can still have threads for deleted/pending/obsolete apps.

**928165**

- On ThreadSerializer, fetch the version number of the thread so we don't have to call out to the version API when we want it.
- On ThreadViewSet list for the per-app dashboard view, expose a nice clean list of (thread ID, version #) tuples to help with creating navigation
- On ThreadViewSet retrieve, also expose the (thread ID, version #) tuple.

TODO: once the comm note permissions PR lands, need to refactor a small part of code to use new code.