Hide unavailable questions

[TaiWilkin]

Overview

While some questions were asked in both years, some questions were only asked in 2020 or 2021. After users select a year, they should only see questions which are applicable in the currently selected year.

Also adds the selected year(s) to the displayed information at the top of each page.

Connects #113

Demo

Testing Instructions

• This is a fully frontend issue, so it can be tested in the Netlify preview.
• Select several countries or regions and the year 2020.
• Navigate to the questions page. Note the number of questions that are available in each category.
• Select questions using the both the individual questions checkboxes and the category checkboxes.
• View the charts page and confirm the selected questions are displayed.
• Return to the years page and select 2021.
• Navigate to the questions page. Note the number of questions that are available in each category, and that they are different from the number of questions available in 2020.
• Repeat the question selection and testing as you did for 2020.

[TaiWilkin]

Thank you for the speedy review!