Squashed 'libertree-db/' changes from a8c1f55..02cf300

02cf300 include English word stems in search indices
35a32dc tagged_posts: operate on hashtags array instead of using regexp
a0cf3b6 store hashtags in an array column
8f1d84e tables for pubsub nodes, subscriptions, affiliations

git-subtree-dir: libertree-db
git-subtree-split: 02cf300645e8bb959c07af9bc772527531232923
Merge commit '4da11e5a9037d744f3356abe375dd81ee43c263e'
update included libertree-db subtree
package up libertree-db

...and make libertree-model depend on it