Add a Mention model

migrations/2018-06-20-175532_create_mentions/down.sql

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

migrations/2018-06-20-175532_create_mentions/up.sql

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+-- Your SQL goes here
+CREATE TABLE mentions (
+    id SERIAL PRIMARY KEY,
+    mentioned_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL,
+    post_id INTEGER REFERENCES posts(id) ON DELETE CASCADE,
+    comment_id INTEGER REFERENCES comments(id) ON DELETE CASCADE
+)