Add "required" property for TextInput, Dropdown, RadioButton, and Checkbox

[natalynyu]

We decided to take advantage of the HTML required property to indicate required form fields in the course admin modal. This property had not been exposed previously. I assigned the value of isRequired to the required property of the TextInput, Dropdown, RadioButton, and Checkbox components.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have run eslint on the code
• [x] I have added JSDoc for all of my code (where applicable)
• [ ] I have added tests to cover my changes.

Priority:

• [x] Normal
• [ ] High

Related Issues:

Addresses #229

[codecov[bot]]

Codecov Report

Merging #84 (0312178) into develop (0a2df14) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #84   +/-   ##
========================================
  Coverage    98.24%   98.24%           
========================================
  Files           42       42           
  Lines          514      514           
  Branches        96       96           
========================================
  Hits           505      505           
  Misses           5        5           
  Partials         4        4           

Impacted Files	Coverage Δ
src/Forms/Checkbox.tsx	100.00% <ø> (ø)
src/Forms/Dropdown.tsx	100.00% <ø> (ø)
src/Forms/RadioButton.tsx	96.00% <ø> (ø)
src/Forms/TextInput.tsx	100.00% <ø> (ø)

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[natalynyu]

@jonseitz That's a good point, thank you. I added the required property to the other form field elements in 0312178.