Feat/notifications table (#3184)

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Creates the `notifications` table and a `user_notifications` join table 

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Closes #
[1-720](https://linear.app/unleash/issue/1-720/db-table-for-notifications-and-real-store-with-link-to-event-store-to)

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---------

Signed-off-by: andreas-unleash <andreas@getunleash.ai>

src/migrations/20230222154915-create-notiications-table.js

@@ -0,0 +1,32 @@
+'use strict';
+
+exports.up = function (db, cb) {
+    db.runSql(
+        `
+            CREATE TABLE IF NOT EXISTS notifications (
+                 id SERIAL PRIMARY KEY,
+                 event_id INTEGER NOT NULL REFERENCES  events (id),
+                 created_by INTEGER NOT NULL REFERENCES  users (id),
+                 created_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT now()
+            );
+
+            CREATE TABLE if not exists user_notifications
+            (
+                notification_id INTEGER NOT NULL REFERENCES notifications (id),
+                user_id         INTEGER NOT NULL REFERENCES  users (id),
+                read_at         TIMESTAMP WITH TIME ZONE,
+                PRIMARY KEY (notification_id, user_id)
+            );
+        `,
+        cb,
+    );
+};
+
+exports.down = function (db, cb) {
+    db.runSql(
+        `
+        DROP TABLE IF EXISTS notifications;
+        DROP TABLE IF EXISTS user_notifications;`,
+        cb,
+    );
+};