add sqlite migrations

we move our PostgreSQL specific migrations to a subdirectory.
The SQLite migrations have been created by running `diesel` against a
copy, and then fixing what's broken.
In the end i reduced all modifications to a single create, since we
*are* starting out fresh with SQLite.

n.b.: i'm not entirely happy with the results yet, because diesel
heavily modifies our `plume-models/src/schema.rs`.
I'll keep fiddling until we have the same types between the two
databases.

migrations/postgres/2018-09-06-182637_posts_add_source/down.sql

@@ -0,0 +1,2 @@
+-- This file should undo anything in `up.sql`
+ALTER TABLE posts DROP COLUMN source;

migrations/postgres/2018-09-06-182637_posts_add_source/up.sql

@@ -0,0 +1,2 @@
+-- Your SQL goes here
+ALTER TABLE posts ADD COLUMN source TEXT NOT NULL DEFAULT '';