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/sqlite/2018-05-09-192013_create_comments/down.sql

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

migrations/sqlite/2018-05-09-192013_create_comments/up.sql

@@ -0,0 +1,12 @@
+-- Your SQL goes here
+CREATE TABLE comments (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    content TEXT NOT NULL DEFAULT '',
+    in_response_to_id INTEGER REFERENCES comments(id),    
+    post_id INTEGER REFERENCES posts(id) ON DELETE CASCADE NOT NULL,
+    author_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL,
+    creation_date INTEGER NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    ap_url VARCHAR,
+    sensitive BOOLEAN NOT NULL DEFAULT 'f',
+    spoiler_text TEXT NOT NULL DEFAULT ''
+)