feat(web/converse): Import repository

Imports the converse forum software I wrote a few years ago. I want to
clean this up a bit and try using Hotwire with it.

Note: The original repository was AGPL-3.0 licensed. I'm the copyright
holder and have relicensed it to GPL-3.0 in the commit that is being
merged.

Imported from: https://github.com/tazjin/converse

git-subtree-dir: web/converse
git-subtree-mainline: 386afdc794
git-subtree-split: 09168021e7
Change-Id: Ia8b587db5174ef5b3c52910d3d027199150c58e0

web/converse/migrations/2018-04-14-170750_search-index/up.sql

@@ -0,0 +1,21 @@
+-- Prepare a materialised view containing the tsvector data for all
+-- threads and posts. This view is indexed using a GIN-index to enable
+-- performant full-text searches.
+--
+-- For now the query language is hardcoded to be English.
+
+CREATE MATERIALIZED VIEW search_index AS
+  SELECT p.id AS post_id,
+         p.author_name AS author,
+         t.id AS thread_id,
+         t.title AS title,
+         p.body AS body,
+         setweight(to_tsvector('english', t.title), 'B') ||
+         setweight(to_tsvector('english', p.body), 'A') ||
+         setweight(to_tsvector('simple', t.author_name), 'C') ||
+         setweight(to_tsvector('simple', p.author_name), 'C') AS document
+    FROM posts p
+    JOIN threads t
+    ON t.id = p.thread_id;
+
+CREATE INDEX idx_fts_search ON search_index USING gin(document);