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/2018-04-22-151330_create_user/down.sql

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

migrations/postgres/2018-04-22-151330_create_user/up.sql

@@ -0,0 +1,13 @@
+-- Your SQL goes here
+CREATE TABLE users (
+    id SERIAL PRIMARY KEY,
+    username VARCHAR NOT NULL,
+    display_name VARCHAR NOT NULL DEFAULT '',
+    outbox_url VARCHAR NOT NULL,
+    inbox_url VARCHAR NOT NULL,
+    is_admin BOOLEAN NOT NULL DEFAULT 'f',
+    summary TEXT NOT NULL DEFAULT '',
+    email TEXT,
+    hashed_password TEXT,
+    instance_id INTEGER REFERENCES instances(id) ON DELETE CASCADE NOT NULL
+);