Review and address code quality issues flagged by SonarCloud

[osma]

Some months ago we added SonarCloud to the set of code analysis tools (in addition to Code Climate, Scrutinizer etc.). The current report shows a number of issues, including 10 classified as bugs, 11 security hotspots and 514 code smells.

These should be reviewed and at least the low hanging fruit and/or uncontroversial issues fixed.

This would be a good introductory task for a person who is not very familiar with the codebase. Many of the flagged issues should be simple to fix without deep knowledge of the code.