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.

diesel.toml

@@ -0,0 +1,5 @@
+# For documentation on how to configure this file,
+# see diesel.rs/guides/configuring-diesel-cli
+
+[print_schema]
+file = "plume-models/src/schema.rs"