Course Planner "Plan to Take" and "Exemptions" sections don't have course autofill feature

Describe the bug

The "Plan to Take" and "Exemptions" sections don't have the course autofill feature as Year X Semester X sections

To Reproduce

Click the "+ Add Courses" buttons in Exemptions and Plan to Take sections
Type in for example "cs", "ma' or things like that
No completion suggestions will show up for users to autofill.

Expected behavior

Should pop up completion suggestions menu for users to autofill the course, which is the behavior when testing in the above "Year X Semester X" sections

Desktop (please complete the following information):

Browser: Brave

This probably has something to do with wrong props being passed to the PlannerModuleSelect component.

nusmods/website/src/views/planner/AddModule.tsx

Lines 102 to 108 in 089993f

    
           <PlannerModuleSelect 
        
             id={inputId} 
        
             rows={3} 
        
             semester={this.props.semester} 
        
             onSelect={this.onSelectModule} 
        
             onCancel={this.onCancel} 
        
           />

I'm labelling this as a good first issue since this seems like a simple UI bug!

I think a tangential issue that can be solved in the same PR is the confusion (because there's no dropdown) that the Add Course button here should be adding the course I enter in the text input.

Perhaps something even better would be renaming the text in the button from "Add Course" to "Add Category" or something that avoids this confusion entirely.

+1 to renaming to "Add Category", I think that's better than adding from the text

Hi I am interested in taking this issue

	<PlannerModuleSelect
	id={inputId}
	rows={3}
	semester={this.props.semester}
	onSelect={this.onSelectModule}
	onCancel={this.onCancel}
	/>