SkelVanilla with a11y and options

Based on vanilla theme, improve accessibility and usuability.

By default using a contrasted css, include booswatch theme.

Include a lot of options

• Allow to set question text to h2 or h3 for better a11y and navigation
• Keyboard navigation improvment
• Allow to choose between well or panel presentation for question (or none).
• Allow to use boostrap helper color for questio text and help.

If you want to create your own variation, you can use https://gitlab.com/SondagesPro/SurveyThemes/less-limesurvey