Create user, post, comment table for dev environment

bytebase/dev/202106280000__blog__baseline__add_initial_schema.sql

+CREATE TABLE `user` (
+	id INTEGER PRIMARY KEY AUTO_INCREMENT,
+	name TEXT NOT NULL,
+	email TEXT NOT NULL
+);
+
+CREATE TABLE post (
+	id INTEGER PRIMARY KEY AUTO_INCREMENT,
+	name TEXT NOT NULL,
+	content TEXT NOT NULL,
+	author_id INTEGER NOT NULL,
+	FOREIGN KEY (author_id) REFERENCES `user` (id) ON DELETE RESTRICT
+);
+
+CREATE TABLE comment (
+	id INTEGER PRIMARY KEY AUTO_INCREMENT,
+	content TEXT NOT NULL,
+	post_id INTEGER NOT NULL,
+	author_id INTEGER NOT NULL,
+	FOREIGN KEY (post_id) REFERENCES post (id) ON DELETE CASCADE,
+	FOREIGN KEY (author_id) REFERENCES `user` (id) ON DELETE RESTRICT
+);