Added database migrations for blocks table (#548)

Added database migrations for `blocks` table

ref https://linear.app/ghost/issue/AP-1082

Added a database table that will store a record of an account block. We will
later query this table to determine if an account has blocked another account /
is being blocked by another account

Author: mike182uk

migrate/migrations/000040_add-blocks-table.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS blocks;

migrate/migrations/000040_add-blocks-table.up.sql

@@ -0,0 +1,14 @@
+CREATE TABLE blocks (
+    id INT UNSIGNED AUTO_INCREMENT PRIMARY KEY,
+    created_at TIMESTAMP(6) NULL DEFAULT CURRENT_TIMESTAMP(6),
+
+    blocker_id INT UNSIGNED NOT NULL,
+    blocked_id INT UNSIGNED NOT NULL,
+
+    UNIQUE KEY unique_blocker_blocked (blocker_id, blocked_id),
+
+    FOREIGN KEY (blocker_id) REFERENCES accounts(id) ON DELETE CASCADE ON UPDATE CASCADE,
+    FOREIGN KEY (blocked_id) REFERENCES accounts(id) ON DELETE CASCADE ON UPDATE CASCADE,
+
+    KEY idx_blocks_blocked_id_blocker_id (blocked_id, blocker_id)
+);