Implement validation rules in backend (#42)

* Move `polling_station ` structs to new module

* Update Cargo dependencies

* Update `openapi.json` using Lefthook

* Implement validation of polling station data entry

* Always return JSON response from data entry API

* Warn when using unwrap in non-test code

And remove Hello World API endpoint.

* Move lint configuration to config files

And also lint tests using Clippy in CI.

* Expand validation integration test assertions

.github/workflows/backend.yml

@@ -40,6 +40,6 @@ jobs:
       - name: Check rustfmt
         run: cargo fmt --all -- --check
       - name: Check clippy
-        run: cargo clippy -- -D warnings
+        run: cargo clippy --all-targets --all-features -- -D warnings
       - name: Run tests
         run: cargo test --verbose