Add blog model

migrations/2018-04-23-101717_create_blogs/down.sql

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+-- This file should undo anything in `up.sql`
+DROP TABLE blogs;

migrations/2018-04-23-101717_create_blogs/up.sql

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+-- Your SQL goes here
+CREATE TABLE blogs (
+    id SERIAL PRIMARY KEY,
+    actor_id VARCHAR NOT NULL,
+    title VARCHAR NOT NULL,
+    summary TEXT NOT NULL DEFAULT '',
+    outbox_url VARCHAR NOT NULL,
+    inbox_url VARCHAR NOT NULL,
+    instance_id INTEGER REFERENCES instances(id) ON DELETE CASCADE NOT NULL
+)