feat(migrations): Add a view for simplified post querying

Adds a view to avoid having to query and join the users & posts table
inside of the application (which isn't particularly convenient in Diesel).

migrations/2018-05-01-183232_simplified-post-view/down.sql

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+DROP VIEW simple_posts;

migrations/2018-05-01-183232_simplified-post-view/up.sql

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+-- Creates a view for listing posts akin to the post table before
+-- splitting out users. This exists to avoid having to do joining
+-- logic and such inside of the application.
+
+CREATE VIEW simple_posts AS
+  SELECT p.id AS id,
+         thread_id, body, posted, user_id,
+         users.name AS author_name,
+         users.email AS author_email
+  FROM posts p
+  JOIN users ON users.id = p.user_id;