Feat: add diesel-rs

- Remove sqlx
- Install diesel-rs and deadpool-diesel
- Re-write migration files
- Add mods `schema` and `models`

## Test Plan

- Install postgresql and edit `DATABASE_URL` in `.env`
- Run the server
- Send **POST** http://localhost:8080/test/create_configs
- GET http://localhost:8080/test/get_configs

.env.example

@@ -1,3 +1,3 @@
-# see https://docs.rs/sqlx/latest/sqlx/mysql/struct.MySqlConnectOptions.html
-DATABASE_URL=mysql://username:password@localhost/rune?socket=/tmp/mysql.sock
+# https://www.postgresql.org/docs/current/libpq-connect.html#LIBPQ-CONNSTRING
+DATABASE_URL=postgres://username:password@localhost/rune
 SERVE_DIR=frontend/dist