Add disabled property to Dropdown and onClick property to Dropdown & TextInput

[natalynyu]

In the Course Planner Course Admin modal, we would like to have the "Select an existing area" dropdown to be disabled when the user is interacting with the "Create New Area" text input field, so I've exposed the disabled property on the Dropdown component. Also, it was noted in our retrospective that it would be nice to have the radio button selected when one of the elements were clicked (e.g. have the Create New Area radio button selected when the text input field is clicked), so I also included an optional onClick property for the TextInput and Dropdown.

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)
• [x] I have added tests to cover my changes.

Priority:

• [x] Normal
• [ ] High

Related Issues:

Addresses #272

[codecov[bot]]

Codecov Report

Merging #82 (2942225) into develop (4fd23e8) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop      #82   +/-   ##
========================================
  Coverage    98.24%   98.24%           
========================================
  Files           42       42           
  Lines          513      513           
  Branches        96       96           
========================================
  Hits           504      504           
  Misses           5        5           
  Partials         4        4           

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

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 71423d4...2942225. Read the comment docs.