UNION attack SQLI challenge now solvable
removed "solvable" column from Challenges (resolves #12)

- all challenges are now solvable
- no need any more to distinguish on UI
disabled SQL logging by default

looking at the logs gives away too many hints for challenges
detailed test description
completed server-side tests

- tests cover all challenged now
- only non-covered paths are some .error() calls and abnormal server exit