Fix remaining Sonarqube major bugs identified

[pjmonks]

The Sonarqube report for mdm-ui can be found here: https://jenkins.cs.ox.ac.uk/sonarqube/project/issues?id=mdm-ui&resolved=false&severities=MAJOR&types=BUG

This shows there are (at the time of writing) 7 major bugs remaining:

• The ones for rule "Remove this conditional structure or edit its code blocks so that they're not all the same." are actually tied to issue #515 so once that is completed these will disappear
• The others should be easily fixable

Removing these final major issues will upgrade the Sonarqube reliability grade from a C to higher.