database abstraction and user authentication

Cargo.lock

Cargo.toml

aava.sql

crates/back/Cargo.toml

crates/back/src/api/auth.rs

crates/back/src/api/course.rs

crates/back/src/api/courses.rs

crates/back/src/api/exercise.rs

crates/back/src/api/lesson.rs

crates/back/src/api/mod.rs

crates/back/src/cli.rs

crates/back/src/db/any.rs

crates/back/src/db/embedded.rs

crates/back/src/db/mod.rs

crates/back/src/db/postgres.rs

crates/back/src/db/sled.rs

crates/back/src/util/jwt.rs

crates/back/src/util/mod.rs