Squashed 'libertree-model-rb/' changes from e840d12..bbe0da8

bbe0da8 Gemfile: drop nokogiri version constraint
acffa9b Gemfile: move json to extensions group

git-subtree-dir: libertree-model-rb
git-subtree-split: bbe0da8ddd932f975cf65aae1eaafb202d9bd603
Merge commit 'c174b7f24a4c78d59e9b432170062d8cf822ac0f'
add recipe for libertree-frontend package