Deactivate expiry of anti-CSRF tokens

If the tokens expire after a short amount of time, users permanently run into trouble when trying to submit a form. In the worst case, they write a long text and then lose everything due to an expired token. This is obviously a problem.

It also doesn't make a lot of sense from a security perspective. The anti-CSRF tokens are less important than the session ID, so there's no reason for rotating them more often.
Fixed error handling of database class

Currently, the code uses different indicators to determine failed queries. For example, an UPDATE query with no affected rows will count as an error, which doesn't make a lot of sense.

It's better to check for real SQL errors. This is done by telling MySQLi to throw exceptions in case of an error. Those exceptions can then be caught and replaced with custom error messages.
Fix error handling of INSERT and DELETE queries
Merge pull request #11 from Jacques1/csrf-token-expiry

Deactivate expiry of anti-CSRF tokens
Merge pull request #12 from Jacques1/mysqli-error-handling

Mysqli error handling