Improve error handling

[constantingoeldel]

• Adds error handling with a common Error struct based on crate thiserror
• introduces .env file
• cargo clippy lint
• some small utility functions

.env Should look like

DATABASE_URL=postgres://...@localhost:5432/anki
ROCKET_SECRET_KEY=

There are two comments starting with REVIEW, in one case, I believe the function can be deleted, in the other, it can be simplified. Do you agree?