bug 1057294 - remove extra kuma-lib submodule layer

Supersedes https://github.com/mozilla/kuma/pull/2869 with rebase and docs update.

Spot-check:
```
rm -rf vendor
git checkout groovecoder/rebased-kuma-lib-1057294
git submodule update --init --recursive
./manage.py test --noinput -v2 access actioncounters authkeys contentflagging devmo kpi kuma landing sumo
```