Form Validation for Create Unit, Create Lesson, and Add Vocab Item modals

[ashayp22]

Describe the bug We should validate the forms on the Create Unit, Create Lesson, and Add Vocab Item modals. We can do this by:

1. Disabling the submit button (can gray out like on the Apply to Contribute page)
2. Adding a * next to required fields

Check out client/src/pages/Apply for examples on how to do the form validation.

[rustom]

Are the validate() function and FormControl components in Apply.js necessary (and corresponding components I'd add to CreateUnit, etc.)? It seems sufficient to just disable the submit button by creating the areAllFilled bool and passing it to the Drawer component in CreateUnit.js, like this:

const areAllFilled = name !== '' && purpose !== ''

return (
    <Drawer
      titleText="Add Custom Unit"
      successText="Create Unit"
      successCallback={success}
      closeCallback={close}
      isDisabled={!areAllFilled}
      body={body}
    />
  )

The same can be done for lesson and vocab item.