Added check if user can manage options to prevent privilege escalation

Missed capabilities checks, leading to authenticated, stored cross-site scripting (XSS) and potentially privilege escalation.

This vulnerability affects the following application versions:

• ThemeGrill Demo Importer 1.3.4
• ThemeGrill Demo Importer 1.4.0
• ThemeGrill Demo Importer 1.4.1
• ThemeGrill Demo Importer 1.4.2
• ThemeGrill Demo Importer 1.4.3
• ThemeGrill Demo Importer 1.5.0
• ThemeGrill Demo Importer 1.5.1
• ThemeGrill Demo Importer 1.5.2
• ThemeGrill Demo Importer 1.5.3
• ThemeGrill Demo Importer 1.5.4
• ThemeGrill Demo Importer 1.5.5
• ThemeGrill Demo Importer 1.5.6
• ThemeGrill Demo Importer 1.5.7
• ThemeGrill Demo Importer 1.5.8
• ThemeGrill Demo Importer 1.5.9
• ThemeGrill Demo Importer 1.6.0
• ThemeGrill Demo Importer 1.6.1