feat(migrations): Drop 'body' column from threads

As a data model simplification, the text of a thread's top post should
simply also go into the posts table.

migrations/2018-04-14-140818_posts_only_in_posts/down.sql

@@ -0,0 +1 @@
+ALTER TABLE threads ADD COLUMN body TEXT NOT NULL DEFAULT '';

migrations/2018-04-14-140818_posts_only_in_posts/up.sql

@@ -0,0 +1,6 @@
+-- Instead of storing the thread OP in the thread table, this will
+-- make it a post as well.
+-- At the time at which this migration was created no important data
+-- existed in any converse instances, so data is not moved.
+
+ALTER TABLE threads DROP COLUMN body;