Implementing ESLint suggestions

src/js/DefaultRules.js

src/js/ageAdjustYoungRelative.js

src/js/assayJS.js

src/js/assignJS.js

src/js/customizeSlaveTrade.js

src/js/datatypeCleanupJS.js

src/js/descriptionWidgets.js

src/js/economyJS.js

src/js/eventSelectionJS.js

src/js/generateGenetics.js

src/js/generateNewSlaveJS.js

src/js/itemAvailability.js

src/js/modification.js

src/js/playerJS.js

src/js/pregJS.js

src/js/rulesAssistant.js

src/js/rulesAutosurgery.js

src/js/slaveCostJS.js

src/js/slaveListing.js

src/js/slaveStatsChecker.js